* Convert README from Markdown to ReStructured Text and use as longdesc
Discussion in #13758 led to deciding to switch README to rst and having
setup.py consume it as the long_description.
* Fix long string in setup.py for pep8 compliance
* Open README.rst as read-only
* Update usages of root README.md to README.rst
Unsure about the file ./packaging/debian/docs as it (only) contains the
text README.md. I believe it's referencing ./packaging/debian/README.md
but maybe someone who knows debian packaging could review it?
* Pick up fixes that had been merged into README.md after the initial conversion to rst
(cherry picked from commit 7afa34ae8a)
