* Document how to build the doumentation with sphinx
For convinience, I added a Makefile which spares one to memorize the
long sphinx command, or type python setup.py build_sphinx
You simply use `make html` and you will get the docs.
* Render README with markdown properly
conf.py includes some code to work around a bug in
common mark. The markdown is now properly converted to HTML.
* Fix rendering of CONTRIBUTING.md
