docs(hacking+README): clarify that deis CLI requires python 2.7

The README.rst file isn't part of the Sphinx documentation, so I wasn't able to use a reStructuredText include and had to repeat a paragraph.

Closes #2320, #2171. We would like to support Python 3, but any rewriting effort for the `deis` CLI at this point would be better spent porting it to Go.