GitHub doesn't play nice re: displaying files that are symlinked. Hard
links don't work at all. This makes the "main" devguide RST file
CONTRIBUTING.rst and then creates devguide.rst as a symlink.
I've tested this with Sphinx and it works fine for generating
documentation.
This replaces the bash script I originally proposed as a way to safely invoke
unit tests with a xonsh script (we need to eat our own dog food). This also
udpates the documentation.