Add contributor guide

[janosh]

Since less people are familiar with poetry than pip, it would help if contributing.md contained steps on how to setup an editable local install of crystal_toolkit.

[mkhorton]

True, I don't have an issue for this, but the choice to use poetry has not been without issue, but I think we'll stick with it for now.

The two basic strategies are either to:

• Use poetry as intended, with poetry install and then poetry run python your_app.py etc.
• Use poetry itself or a tool like dephell to export a requirements.txt and/or a setup.py, for people who are more comfortable with that (we briefly tried having these alternatives checked into the repo, but it caused more confusion)

[janosh]

Small update: poetry was dropped in favor of pip-tools in https://github.com/materialsproject/crystaltoolkit/pull/292 but would still be good to have a contributing guide for newcomers.