Document our explicit Python version support.

I was reading up on some things (like when Travis dropped Python 2.5) and figured it'd be good to encapsulate this in our docs.

Check out
https://github.com/dhermes/gcloud-python/blob/document-supported-versions/README.rst
for the rendered version.

I wasn't sure if this should go in the README or in CONTRIBUTING. I'd be open to putting the bare bones version listing in the README and then linking to the more esoteric bits in the CONTRIBUTING section.