flesh out contributing guidelines

[jGaboardi]

Flesh out contributing guidelines. Put any ideas here

• xref:
  • Create CONTRIBUTING.md -- document procedure #45 (comment)
  • Add contributing guidelines document #4

[martinfleis]

In the past, I've either copied GeoPandas (momepy) or used this https://github.com/bttger/contributing-gen (clustergram), where I like the latter option much more as a starting point.

[jGaboardi]

Still need:

• name update following resolution of brainstorm package name #64 +++ link updating
• link updating for docs if we do a RTD site -- formal API documentation #5