Update readme and a few other things for release

As per the discussion in #32, this makes some changes to the README to look appropriate for release, as well as a brief ``CONTRIBUTING.md`` that basically just explains the astropy/astropy-helpers relationship. Also, I noticed the year in LICENSE.rst wasn't right so I fixed that.

Ideally, we would also have some instructions in the readme for how to actually use this, but that's probably not too critical for release.  

cc @astrofrog @embray