docs: Add contribution guidelines based on zulip/zulip.

Author: klardotsh

README.md

@@ -6,4 +6,17 @@ This is a collection of GitHub Actions to interact with [Zulip](https://zulip.co
 
 ## Actions
 
-- [Sends a message to Zulip](./send-message/README.md)
+- [`zulip/github-actions-zulip/send-message@v1` sends a message to Zulip](./send-message/README.md)
+
+## Contributing
+
+These actions are implemented in TypeScript, see
+[`tsconfig.json`](./tsconfig.json) for (generally strict mode defaults)
+compiler settings. Use `npm run lint:fix` to autoformat all TypeScript and
+Markdown to the expected style. Use `npm run package` to generate
+`dist/**/*.js`, never edit those files by hand. Commits should follow [the
+Zulip Commit Discipline
+guide](https://zulip.readthedocs.io/en/latest/contributing/version-control.html),
+which can then be contributed upstream to this repository following the [Zulip
+Reviewable Pull Requests
+guide](https://zulip.readthedocs.io/en/latest/contributing/reviewable-prs.html)