Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+## Setup
+
+The project uses [Rye](https://rye-up.com), but feel free to install or setup the required packages using plain `pip` or whatever you prefer.
+
+Install the dependencies using 
+
+```shell
+rye sync
+```
+
+and run the tests using
+
+```shell
+pytest
+```
+
+The project also uses `ruff` as a linter and formatter, so you may use
+
+```shell
+ruff check
+```
+
+to run automated checks and
+
+```shell
+ruff format
+```
+
+to automatically format your code.
+Both of these steps will run automatically in GitHub actions on every PR.
+