Document (in Sphinx) our markdown spec

[ellisonbg]

This is a part of this overall project:

https://github.com/jupyter/nbformat/projects/1

and is a follow on to #95 that will create a sample notebook. In this issue we need to write documentation in the nbformat repo using Sphinx about the markdown we support. That guide should take CommonMark as a starting point and describe what we do and don't support as well as other extensions.

@ashutoshbondre @mpacer

[rgbkrk]

Related to #80.