Add Example Usage to README

Add an example usage case to the README with screenshots and a visible
target and tracking repo (one and the same for example) at
aaron0browne/github-pr-form-tracking Remove the README.rst file as I
can't handle updating the rst in parallel and just use the md file for
the long description...sorry PyPI browsers.

Signed-off-by: Aaron Browne <aaron0browne@gmail.com>