Add CONTRIBUTING.rst.

Addresses #218 partially.

Lifted from https://github.com/Pylons/pyramid/blob/master/HACKING.txt,
and lightly sanded.
Normalize rendering of command examples.

Feedback from @silvolu.
Note PEP exception for _callFUT, MUT.
Merge pull request #219 from tseaver/add-CONTRIBUTING

Add CONTRIBUTING.rst.