Developer guidelines in the docs

This PR changes the docs to add developer guidelines.

it also renames the `Docs` folder to `docs`, to be more consistent with python style. And I added a little bit of stuff to the README

Here's the current developer page on the docs:
https://msmbuilder.readthedocs.org/en/latest/developer.html

Here's the new version:
https://rmcgibbo-msmbuilder.readthedocs.org/en/latest/developer.html

90% of the content is copied from here: http://scikit-learn.org/stable/developers/index.html