docs: create CONTRIBUTING.md

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+# How to contribute
+
+`focus` development is done entirely on Github. To discuss, use the [issue tracker](https://github.com/twitter/focus/issues) or [discussion board](https://github.com/twitter/focus/discussions).
+
+To contribute, open a new pull request against the `main` branch. As part of the pull request, you will be prompted by a bot to sign our CLA if you haven't done so already.
+
+To build and test the code, run
+
+```sh
+$ cargo test --workspace
+```
+
+If you create a new file, you should add a license header. (This will be checked in CI.) You can do this with the following script:
+
+```sh
+$ scripts/add-license.sh
+```