wrong link for "Edit on GitHub" of README.rst (if index.rst does not exist)

[wdv4758h]

Currently, doc_builder/base.py will rename README.rst to index.rst if repo does not have index.rst. And this make the generated index.html finding index.rst as source. Then, it cause 404 when user click "Edit on GitHub". (repo only have README.rst)

[gregmuellegger]

Example: http://python-best-practices.readthedocs.org/en/latest/

[ericholscher]

I believe our servers will now serve README.html as an index file, so we can probably stop transforming the files and be good.

[gregmuellegger]

@ericholscher I tried this in #1585, but it seems to be more complicated. See there for comments.