Add CONTRIBUTING.md

Author: JonasKs

CONTRIBUTING.md

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+# Contributing
+
+This package is open to contributions 👏
+
+To contribute, please follow these steps:
+
+1. Create an issue explaining what you'd like to fix or add. This way, we can approve and discuss the 
+solution before any time is spent on developing it.
+2. Fork the upstream repository into a personal account.
+3. Install [poetry](https://python-poetry.org/), and install all dependencies using ``poetry install``
+4. Activate the environment by running ``poetry shell``
+5. Install [pre-commit](https://pre-commit.com/) (for project linting) by running ``pre-commit install``
+6. Create a new branch for your changes, and make sure to add tests!
+7. Push the topic branch to your personal fork
+8. Run `pre-commit run --all-files` locally to ensure proper linting
+9. Create a pull request to the Intility repository with a detailed summary of your changes and what motivated the change
+
+If you need a more detailed walk through, please see this 
+[issue comment](https://github.com/Intility/fastapi-azure-auth/issues/49#issuecomment-1056962282).