rst: add github-specific comment to visit readthedocs

[SteVwonder]

Github's markup render maintainers have said they will not support Sphinx-specific extensions to ReST, including :doc: and :toctree:.

Reading the comments in the markup source code, GitHub suggests that we add a comment at the top of the file saying the user should visit readTheDocs for the nicely rendered file. Poking at their renderer's source code, it looks like if you start a comment with github display, then github will show the comment in the render. Source.