Create guide for contributors

Write documentation on how to set up an environment for people looking to contribute to the project. We should assume the person looking to contribute doesn't have python or git installed and will guide them as if they just got a fresh computer with nothing installed.

Topics to cover
- Installing python 3.10
- Installing git
- Basic use of git
- Set up virtual environment for Relay repo
- Set up pre-commit
- Running the app
- Using / Testing the app