Add a CONTRIBUTING.md

[dlrobertson]

We should add a CONTRIBUTING.md to this project with the "rules of the road" for contributing here and "were to start" for new contributors. Perhaps "filling out" the issue labels should be part of this as well. Also @blakeflei we may want to have a general template for anidata projects?

CC @gwpjp @blakeflei @egrossman

[wdm0006]

Rasp has a first pass at a contributing file, that may be useful as a example here (the relevant parts at least): https://github.com/anidata/rasp/blob/master/CONTRIBUTING.md