Add README

README.md

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-# build-zeek
+# build-zeek
+
+Build [Zeek](https://zeek.org/) for packaging with
+[Brimcap](https://github.com/brimdata/brimcap) and
+[Zui](https://zui.brimdata.io/).
+
+## Background
+
+Before there was official support for compiling Zeek on Windows, developers
+at Brim Data created a [working port](https://github.com/brimdata/zeek) and an
+[artifact based on Zeek v3.2.1](https://github.com/brimdata/zeek/releases/tag/v3.2.1-brim10).
+Because the effort of keeping the port in sync with ongoing Zeek development
+would have been prohibitive, that artifact shipped with Brimcap and Zui for
+years.
+
+In late 2022, work began to officially support
+[Zeek on Windows](https://zeek.org/2022/11/28/zeek-on-windows/),
+and in late 2023 this repo was created to take advantage of that. The minimal
+glue found here starts from the official Zeek source code and makes only the
+changes necessary to build in GitHub Actions, add some needed
+[Zeek Packages](https://packages.zeek.org/), and produce artifacts ready for
+use in Brimcap/Zui.
+
+## Usage
+
+Create artifacts using the **Create Zeek release** workflow.
+
+![Create Zeek release workflow](workflow.png)
+
+If the given `release_tag` starts with a `v` the artifacts will be published
+to the **Releases** area of this repo. If not, the artifacts will just be
+attached to the Actions run.