Update docs to explain things around pip requirements

[machallboyd]

Following PR https://github.com/NREL/mappymatch/pull/140, we've got requirements defined suitable for installation with pip, including one set for contributor installs.

• Explain pip contributor install for non-conda contributors.
• Explain how to add new requirements if contributing. (Add to setup.py)
• Explain how to use the pip-compile command from pip-tools to automatically generate new pinned requirements. (Covers generating from setup.py for main requirements and from requirements.in files for other requirement sets)

[nreinicke]

For the time being, I've rolled backed our pinned requirements to keep things a bit simpler and I've added a new section into the contributor docs for how to add a new dependency (basically just add to the pyproject.toml file).