Add contributor guidelines

[avital]

No description provided.

[curran]

Having contributor guidelines would definitely be useful. I have made a number of pull requests, and I'm not sure why they are not being reviewed or merged. My guess is that it's a lack of time on the part of the maintainers, but I also wonder if I'm doing something wrong, or if something more is expected from outside contributors like me (e.g. a detailed explanation of why the change is required). Having clarity on what's expected of contributors would be great.

[curran]

I suggest we close this in favor of #240

[alecgibson]

Agreed with @curran