Add tip about local virtual environment location config to contributing.md

[kevinschaper]

It's really helpful in poetry to have your virtual environment get created in a consistent place, so that the IDE always know where to point, even if you might want to clobber the environment and start over.

We could add a suggestion in our contributing.md about using

poetry config virtualenvs.in-project true

[glass-ships]

looks like this is already in the documentation