[readme] add content

README.md

@@ -1,2 +1,61 @@
-# side-channel
+# side-channel <sup>[![Version Badge][npm-version-svg]][package-url]</sup>
+
+[![github actions][actions-image]][actions-url]
+[![coverage][codecov-image]][codecov-url]
+[![License][license-image]][license-url]
+[![Downloads][downloads-image]][downloads-url]
+
+[![npm badge][npm-badge-png]][package-url]
+
 Store information about any JS value in a side channel. Uses WeakMap if available.
+
+Warning: in an environment that lacks `WeakMap`, this implementation will leak memory until you `delete` the `key`.
+
+## Getting started
+
+```sh
+npm install --save side-channel
+```
+
+## Usage/Examples
+
+```js
+const assert = require('assert');
+const getSideChannel = require('side-channel');
+
+const channel = getSideChannel();
+
+const key = {};
+assert.equal(channel.has(key), false);
+assert.throws(() => channel.assert(key), TypeError);
+
+channel.set(key, 42);
+
+channel.assert(key); // does not throw
+assert.equal(channel.has(key), true);
+assert.equal(channel.get(key), 42);
+
+channel.delete(key);
+assert.equal(channel.has(key), false);
+assert.throws(() => channel.assert(key), TypeError);
+```
+
+## Tests
+
+Clone the repo, `npm install`, and run `npm test`
+
+[package-url]: https://npmjs.org/package/side-channel
+[npm-version-svg]: https://versionbadg.es/ljharb/side-channel.svg
+[deps-svg]: https://david-dm.org/ljharb/side-channel.svg
+[deps-url]: https://david-dm.org/ljharb/side-channel
+[dev-deps-svg]: https://david-dm.org/ljharb/side-channel/dev-status.svg
+[dev-deps-url]: https://david-dm.org/ljharb/side-channel#info=devDependencies
+[npm-badge-png]: https://nodei.co/npm/side-channel.png?downloads=true&stars=true
+[license-image]: https://img.shields.io/npm/l/side-channel.svg
+[license-url]: LICENSE
+[downloads-image]: https://img.shields.io/npm/dm/side-channel.svg
+[downloads-url]: https://npm-stat.com/charts.html?package=side-channel
+[codecov-image]: https://codecov.io/gh/ljharb/side-channel/branch/main/graphs/badge.svg
+[codecov-url]: https://app.codecov.io/gh/ljharb/side-channel/
+[actions-image]: https://img.shields.io/endpoint?url=https://github-actions-badge-u3jn4tfpocch.runkit.sh/ljharb/side-channel
+[actions-url]: https://github.com/ljharb/side-channel/actions