docs: create CONTRIBUTING.md

Author: EndBug

CONTRIBUTING.md

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+# Contributing
+
+If you want to contribute to this project, check out this steps!
+
+1. Check out existing features to make sure your case is not already covered. Also, try [searching open or closed issues](https://github.com/EndBug/add-and-commit/issues) that may cover the same topic.
+2. Either [open a new issue](https://github.com/EndBug/add-and-commit/issues/new/choose) or comment on an existing one to let everyone know what you're working on.
+3. Edit the source files to implement your feature or fix.
+4. Build the action and test it in a test repo.
+5. Update the [action manifest](./action.yml) AND the [README](./README.md) with your changes. 
+6. [Open a PR](https://github.com/EndBug/add-and-commit/compare).
+
+Thanks! 💖
+