Make it a policy to include the text of legal tools

In a future commit, I plan on adding a sound effect to this project. I’m
not going to make the sound effect myself. Instead, I’m going to find
one that’s either available under a free license or in the public
domain. It’s more likely that I’ll end up finding a freely licensed one
than a public domain one.

If I do end up finding something that’s available under a free license,
then that license will probably require that I include a copy of it. For
the sake of consistency, this commit makes it a policy to include a copy
of every legal tool that at least some content in this repository is
available under.

Since this repo already contains CC0’d content, this commit adds a copy
of CC0. While progress has been made [1] into closing the
CC0-not-available-under-CC0 loophole [2] [3], it’s not fully closed.
Creative Commons assigned someone to one of the issues, and that
assignee said that CC0 __should__ already be available under CC0 [1].
This, unfortunately, isn’t quite the same as Creative Commons outright
saying that CC0 __is__ available under CC0. Additionally, some text was
added to CC’s Licensing Statement for Content and Software Code [4]
which dedicates “the text of all of the notices and accompanying text on
the license pages” to the public domain using CC0. The key words here
are “on the license pages”. CC0 is not a license [5] so that quote,
unfortunately, doesn’t apply to CC0. Ugh.

With that in mind, I decided to still include attribution text for the
portions of CC0 that are (potentially) CC BY 4.0 licensed. It’s probably
not necessary, but I’d rather err on the side of caution.

[1]: <https://github.com/creativecommons/creativecommons.org/issues/1218#issuecomment-1212525987>
[2]: <creativecommons/creativecommons.org#1212>
[3]: <https://github.com/creativecommons/creativecommons.org/issues/1218>
[4]: <https://creativecommons.org/policies#licensing%20statement>
[5]: <https://creativecommons.org/faq/#how-do-cc-licenses-operate>

CC0-1.0.txt

@@ -0,0 +1,121 @@
+Creative Commons Legal Code
+
+CC0 1.0 Universal
+
+    CREATIVE COMMONS CORPORATION IS NOT A LAW FIRM AND DOES NOT PROVIDE
+    LEGAL SERVICES. DISTRIBUTION OF THIS DOCUMENT DOES NOT CREATE AN
+    ATTORNEY-CLIENT RELATIONSHIP. CREATIVE COMMONS PROVIDES THIS
+    INFORMATION ON AN "AS-IS" BASIS. CREATIVE COMMONS MAKES NO WARRANTIES
+    REGARDING THE USE OF THIS DOCUMENT OR THE INFORMATION OR WORKS
+    PROVIDED HEREUNDER, AND DISCLAIMS LIABILITY FOR DAMAGES RESULTING FROM
+    THE USE OF THIS DOCUMENT OR THE INFORMATION OR WORKS PROVIDED
+    HEREUNDER.
+
+Statement of Purpose
+
+The laws of most jurisdictions throughout the world automatically confer
+exclusive Copyright and Related Rights (defined below) upon the creator
+and subsequent owner(s) (each and all, an "owner") of an original work of
+authorship and/or a database (each, a "Work").
+
+Certain owners wish to permanently relinquish those rights to a Work for
+the purpose of contributing to a commons of creative, cultural and
+scientific works ("Commons") that the public can reliably and without fear
+of later claims of infringement build upon, modify, incorporate in other
+works, reuse and redistribute as freely as possible in any form whatsoever
+and for any purposes, including without limitation commercial purposes.
+These owners may contribute to the Commons to promote the ideal of a free
+culture and the further production of creative, cultural and scientific
+works, or to gain reputation or greater distribution for their Work in
+part through the use and efforts of others.
+
+For these and/or other purposes and motivations, and without any
+expectation of additional consideration or compensation, the person
+associating CC0 with a Work (the "Affirmer"), to the extent that he or she
+is an owner of Copyright and Related Rights in the Work, voluntarily
+elects to apply CC0 to the Work and publicly distribute the Work under its
+terms, with knowledge of his or her Copyright and Related Rights in the
+Work and the meaning and intended legal effect of CC0 on those rights.
+
+1. Copyright and Related Rights. A Work made available under CC0 may be
+protected by copyright and related or neighboring rights ("Copyright and
+Related Rights"). Copyright and Related Rights include, but are not
+limited to, the following:
+
+  i. the right to reproduce, adapt, distribute, perform, display,
+     communicate, and translate a Work;
+ ii. moral rights retained by the original author(s) and/or performer(s);
+iii. publicity and privacy rights pertaining to a person's image or
+     likeness depicted in a Work;
+ iv. rights protecting against unfair competition in regards to a Work,
+     subject to the limitations in paragraph 4(a), below;
+  v. rights protecting the extraction, dissemination, use and reuse of data
+     in a Work;
+ vi. database rights (such as those arising under Directive 96/9/EC of the
+     European Parliament and of the Council of 11 March 1996 on the legal
+     protection of databases, and under any national implementation
+     thereof, including any amended or successor version of such
+     directive); and
+vii. other similar, equivalent or corresponding rights throughout the
+     world based on applicable law or treaty, and any national
+     implementations thereof.
+
+2. Waiver. To the greatest extent permitted by, but not in contravention
+of, applicable law, Affirmer hereby overtly, fully, permanently,
+irrevocably and unconditionally waives, abandons, and surrenders all of
+Affirmer's Copyright and Related Rights and associated claims and causes
+of action, whether now known or unknown (including existing as well as
+future claims and causes of action), in the Work (i) in all territories
+worldwide, (ii) for the maximum duration provided by applicable law or
+treaty (including future time extensions), (iii) in any current or future
+medium and for any number of copies, and (iv) for any purpose whatsoever,
+including without limitation commercial, advertising or promotional
+purposes (the "Waiver"). Affirmer makes the Waiver for the benefit of each
+member of the public at large and to the detriment of Affirmer's heirs and
+successors, fully intending that such Waiver shall not be subject to
+revocation, rescission, cancellation, termination, or any other legal or
+equitable action to disrupt the quiet enjoyment of the Work by the public
+as contemplated by Affirmer's express Statement of Purpose.
+
+3. Public License Fallback. Should any part of the Waiver for any reason
+be judged legally invalid or ineffective under applicable law, then the
+Waiver shall be preserved to the maximum extent permitted taking into
+account Affirmer's express Statement of Purpose. In addition, to the
+extent the Waiver is so judged Affirmer hereby grants to each affected
+person a royalty-free, non transferable, non sublicensable, non exclusive,
+irrevocable and unconditional license to exercise Affirmer's Copyright and
+Related Rights in the Work (i) in all territories worldwide, (ii) for the
+maximum duration provided by applicable law or treaty (including future
+time extensions), (iii) in any current or future medium and for any number
+of copies, and (iv) for any purpose whatsoever, including without
+limitation commercial, advertising or promotional purposes (the
+"License"). The License shall be deemed effective as of the date CC0 was
+applied by Affirmer to the Work. Should any part of the License for any
+reason be judged legally invalid or ineffective under applicable law, such
+partial invalidity or ineffectiveness shall not invalidate the remainder
+of the License, and in such case Affirmer hereby affirms that he or she
+will not (i) exercise any of his or her remaining Copyright and Related
+Rights in the Work or (ii) assert any associated claims and causes of
+action with respect to the Work, in either case contrary to Affirmer's
+express Statement of Purpose.
+
+4. Limitations and Disclaimers.
+
+ a. No trademark or patent rights held by Affirmer are waived, abandoned,
+    surrendered, licensed or otherwise affected by this document.
+ b. Affirmer offers the Work as-is and makes no representations or
+    warranties of any kind concerning the Work, express, implied,
+    statutory or otherwise, including without limitation warranties of
+    title, merchantability, fitness for a particular purpose, non
+    infringement, or the absence of latent or other defects, accuracy, or
+    the present or absence of errors, whether or not discoverable, all to
+    the greatest extent permissible under applicable law.
+ c. Affirmer disclaims responsibility for clearing rights of other persons
+    that may apply to the Work or any use thereof, including without
+    limitation any person's Copyright and Related Rights in the Work.
+    Further, Affirmer disclaims responsibility for obtaining any necessary
+    consents, permissions or other rights required for any use of the
+    Work.
+ d. Affirmer understands and acknowledges that Creative Commons is not a
+    party to this document and has no duty or obligation with respect to
+    this CC0 or use of the Work.

CC0-1.0.txt.spdx-meta

@@ -0,0 +1 @@
+SPDX-FileAttributionText: Parts of the “CC0 1.0 Universal (CC0 1.0) Public Domain Dedication” <https://creativecommons.org/publicdomain/zero/1.0/legalcode> by Creative Commons <https://creativecommons.org/> are licensed under CC BY 4.0 <https://creativecommons.org/licenses/by/4.0/>. Parts of the “CC0 1.0 Universal (CC0 1.0) Public Domain Dedication” are available under the CC0 Public Domain Dedication <https://creativecommons.org/publicdomain/zero/1.0/>. More details can be found at <https://github.com/creativecommons/creativecommons.org/issues/1212> and <https://github.com/creativecommons/creativecommons.org/issues/1218>.

COPYING.md

@@ -8,5 +8,68 @@ SPDX-FileContributor: Jason Yundt <[email protected]> (2021–2023)
 This project uses [SPDX license list short identifiers] and [SPDX file tags] to
 declare its copying information.
 
+## SPDX Metadata Files
+
+Normally, [SPDX license list short identifiers] and [SPDX file tags] are put in
+the files they describe. For example, in a comment at the top of this file,
+there’s an [SPDX file tag] which says that “Jason Yundt” [contributed](https://spdx.github.io/spdx-spec/v2.3/file-information/#814-file-contributor-field)
+to this file.
+
+Sometimes, for technical reasons, an [SPDX license list short identifier] or
+[SPDX file tag] can’t be included in the file itself. For example,
+[`Pipfile.lock`](https://pipenv.pypa.io/en/stable/basics/) is a [JSON] file.
+[JSON files can’t have comments](https://stackoverflow.com/questions/244777/can-comments-be-used-in-json),
+so there isn’t a good place to put an [SPDX license list short identifier] or
+[SPDX file tag] in a [JSON] file.
+
+The solution: SPDX Metadata Files. SPDX Metadata Files are [plain text](https://www.rfc-editor.org/rfc/rfc2046.html#section-4.1)
+files who’s names end with “`.spdx-meta`”. Every SPDX Metadata File has a
+companion file. A SPDX Metadata File must be in the same folder as its
+companion file in order to be valid. The SPDX Metadata File’s name must be the
+companion file’s name with “`.spdx-meta`” added to the end. For example,
+consider the following directory tree:
+
+	.
+	├── a
+	│   ├── foo.txt
+	│   └── foo.txt.spdx-meta
+	├── b
+	│   └── bar.txt.spdx-meta
+	├── bar.txt
+	└── bar.txt.spdx-meta
+
+In this example,
+
+- `./a/foo.txt.spdx-meta` is an SPDX Metadata File because its name ends with
+“`.spdx-meta`”. Its companion file is `./a/foo.txt`.
+- `./bar.txt.spdx-meta` is also an SPDX Metadata File. Its companion file is
+`./bar.txt`.
+- `./b/bar.txt.spdx-meta` is an SPDX Metadata File, but it’s an invalid one.
+It’s an SPDX Metadata File because its name ends with “`.spdx-meta`”. It’s
+invalid because it has no companion file.
+
+SPDX Metadata Files describe their companion files; [SPDX license list short
+identifiers] and [SPDX file tags] in SPDX Metadata Files don’t describe the SPDX
+Metadata Files that they’re in.
+
+Consider that previous example again. If `./a/foo.txt.spdx-meta` contained
+“SPDX-FileType” followed by a colon, a space and “DOCUMENTATION”, that would
+mean that `./a/foo.txt` is a documentation file. That piece of information
+alone tells you nothing about `./a/foo.txt.spdx-meta`.
+
+All SPDX Metadata Files in this repo are dedicated to the public domain using
+[the CC0 1.0 Universal Public Domain Dedication].
+
+## Location of legal tools
+
+Any licenses that this repo uses can be found in the directory named
+`LICENSES`. A copy of [the CC0 1.0 Universal Public Domain Dedication] can be
+found in the file named `CC0-1.0.txt`.
+
+[JSON]: https://www.json.org/json-en.html
+[NixOS]: https://nixos.org/
+[SPDX file tag]: https://spdx.github.io/spdx-spec/v2.3/file-tags/
 [SPDX file tags]: https://spdx.github.io/spdx-spec/v2.3/file-tags/
+[SPDX license list short identifier]: https://spdx.github.io/spdx-spec/v2.3/using-SPDX-short-identifiers-in-source-files/
 [SPDX license list short identifiers]: https://spdx.github.io/spdx-spec/v2.3/using-SPDX-short-identifiers-in-source-files/
+[the CC0 1.0 Universal Public Domain Dedication]: https://creativecommons.org/publicdomain/zero/1.0/