Add a contribution guide

[will-langdale]

I've been working through a few issues in the repo and it'd be quite useful to have a small CONTRIBUTING.md to help me get started. Knowing your conventions, prerequisites etc would have saved me a bit of time. Things like:

• A python venv inside py_maplib with the requirements in pt_maplib/tests/requirements.txt is important for pytest
• Some Rust dependencies require the nightly build of Rust (rustup install nightly)

It would also be helpful to have something like ruff in the mix to enforce your style rules.

[magbak]

Hi! Thanks @will-langdale!
Will follow up here in not too long.