Add Read the Docs Docs

Added Docs Using Sphinx, RST, and Read the Docs. 

You can see the first version here: http://django-mangopay.readthedocs.org/en/latest/

Right now its just a slightly updated version of the old README.md with extra template code needed to get ReadtheDocs to work. 

@anah I'm planning on updating the docs more, but I thought I would do a PR now before the change become to massive.