add development guide (#47)

Author: jhuleatt

CONTRIBUTING.md

@@ -27,3 +27,38 @@ information on using pull requests.
 
 This project follows
 [Google's Open Source Community Guidelines](https://opensource.google/conduct/).
+
+## Development guide
+
+### Get set up
+
+1. [Clone](https://help.github.com/en/github/creating-cloning-and-archiving-repositories/cloning-a-repository) this repository (or a [fork](https://help.github.com/en/github/getting-started-with-github/fork-a-repo#propose-changes-to-someone-elses-project)).
+1. At the project root, install all modules by running `npm install`.
+
+### Creating a Pull Request
+
+> Note: You must build from source and check in any changes to the contents of the `bin` directory.
+
+1. Before creating a pull request, run the following commands to lint, build, and test your changes:
+
+   ```shell
+   # run the linter
+   npm run format:check
+
+   # rebuild source
+   npm run build
+
+   # run unit tests
+   npm run test
+   ```
+
+1. If you've forked the repo and want to watch the action run, add secrets to your forked repo that match the secrets specified in [one of the workflow files](https://github.com/FirebaseExtended/action-hosting-deploy/tree/main/.github/workflows) you want to test, and trigger the workflow in your forked repo (for example, by creating a pr or pushing to the `main` branch)
+1. Once you're confident in your changes, create a pull request against the firebaseextended/action-hosting-deploy repo.
+
+## Publishing a new version
+
+A repo owner can publish a new version of the action by following the instructions [in the GitHub docs](https://docs.github.com/en/free-pro-team@latest/actions/creating-actions/publishing-actions-in-github-marketplace#publishing-an-action). Manual releases should follow the convention of our existing releases, like [`v0.4-alpha`](https://github.com/FirebaseExtended/action-hosting-deploy/releases/tag/v0.4-alpha).
+
+The current major version is `v0`. We move this tag up to the latest release using the method [recommended by GitHub](https://github.com/actions/toolkit/blob/master/docs/action-versioning.md#user-content-recommendations:~:text=Make%20the%20new%20release%20available%20to%20those%20binding%20to%20the%20major%20version%20tag).
+
+It is important to note that [firebase-tools references `v0`](https://github.com/firebase/firebase-tools/blob/a1fd2ee6ab2f7b4ac7de021226781f5a8f913e18/src/init/features/hosting/github.ts#L32), so any change in the major version should have a linked firebase-tools PR.