README: add some contributing guidelines

Author: jrfnl

README.md

@@ -13,3 +13,11 @@ This repository is set up to automatically update the <https://schema.phpcodesni
     // Example for the XSD file for PHP_CodeSniffer 3.13.x:
     https://schema.phpcodesniffer.com/3.13/phpcs.xsd
     ```
+
+## Contributing
+
+The script which generates the "website" is a combination between the code in `src` and a workflow in the `.github/workflows` folder.
+The `_site` directory is auto-generated by that script and should never be "manually" updated.
+
+To contribute, fork this repo, create a new branch, make your changes and submit a pull request.
+Or open an issue to discuss the proposed change ahead of the PR.