chore: Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Contributions are always welcome. If you would like to propose
+a major change, it's probably a good idea to open an issue to discuss it first,
+otherwise a PR is fine.
+
+To submit a PR:
+
+1. Fork the project
+2. Make your changes
+3. Add tests
+4. Open the PR
+
+## Running the project locally
+
+A simple way to test things is to do create
+a python file with the syntax you want to test,
+then run `cargo run -- <filename>.py` to run the formatter
+on it. Once you're happy with your changes you can translate
+it to a test input.