Add documentation to explain how to install it.

[kezabelle]

Both the README.rst and the RTD docs reference django-12factor but the published package is called django12factor on PyPI.
So pip install django-12factor doesn't work (and worse, would install the wrong package if someone publishes that into the global package index's namespace) ... and is literally the first thing I tried.
I'd suggest adding a short note to both so that people can get and running with that bit less friction.