Add contributing guidelines for new releases

Added guidelines for releasing a new version of the project.

Author: jordanstephens

CONTRIBUTING.md

@@ -0,0 +1,12 @@
+# Contributing
+
+## Releasing a New Version
+
+Navigate to the [New Release]([https://github.com/DefangLabs/defang-github-action/releases](https://github.com/DefangLabs/defang-github-action/releases/new)) form for this repo.
+
+Make sure `Publish this Action to the GitHub Marketplace` is checked.
+
+* **If you have already cut a tag**, you can choose that tag in the form.
+* **If you have not pushed a tag**, write the name of the tag you want to create, for example `v1.2.3`, and make sure the target is set to the `main` branch.
+
+You do not need to provider a release description, but you should write a release title which starts with the tag name, ex: `v1.2.3 Add support for My New Feature`.