Add CONTRIBUTING.rst.

Addresses #218 partially.

Lifted from https://github.com/Pylons/pyramid/blob/master/HACKING.txt, and lightly sanded.  Maybe a bit of a strawman, but the Pyramid version has helped new contributors get started in the past.
I'd like to be consistent in the way we show commands inside the doc, can we always have it like this list (black on gray background)