add a CONTRIBUTING file

CONTRIBUTING.md

@@ -0,0 +1,32 @@
+# Contributing 💻
+
+All contributions are welcome and greatly appreciated!
+
+## Steps to Contribute 💡
+
+> Check the `.node-version` file in the root of this repo so see what version of Node.js is required for local development - note, this can be different from the version of Node.js which runs the Action on GitHub runners. It is suggested to download [nodenv](https://github.com/nodenv/nodenv) which uses this file and manages your Node.js versions for you
+
+1. Fork this repository
+2. Commit your changes
+3. Test your changes (learn how to test below)
+4. Open a pull request back to this repository
+5. Notify the maintainers of this repository for peer review and approval
+6. Merge!
+
+The maintainers of this repository will create a new release with your changes so that everyone can use the new release and enjoy the awesome features of this Action!
+
+## Testing 🧪
+
+This project requires **100%** test coverage
+
+> The deploy-pages Action is used by enterprises, governments, and open source organizations - it is critical that we have 100% test coverage to ensure that we are not introducing any regressions. All changes will be throughly tested by maintainers of this repository before a new release is created.
+
+### Running the test suite (required)
+
+Simply run the following command to execute the entire test suite:
+
+```bash
+npm run test
+```
+
+> Note: this requires that you have already run `npm install`