Convert README from Markdown to ReStructured Text and use as longdesc (#22330)

* 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

README.rst

@@ -0,0 +1,104 @@
+|PyPI version| |Build Status|
+
+*******
+Ansible
+*******
+
+Ansible is a radically simple IT automation system. It handles
+configuration-management, application deployment, cloud provisioning,
+ad-hoc task-execution, and multinode orchestration -- including
+trivializing things like zero-downtime rolling updates with load
+balancers.
+
+Read the documentation and more at https://ansible.com/
+
+You can find installation instructions
+`here <https://docs.ansible.com/intro_getting_started.html>`_ for a
+variety of platforms.
+
+Most users should probably install a released version of Ansible from ``pip``, a package manager or
+our `release repository <https://releases.ansible.com/ansible/>`_. `Officially supported
+<https://www.ansible.com/ansible-engine>`_ builds of Ansible are also available. Some power users
+run directly from the development branch - while significant efforts are made to ensure that
+``devel`` is reasonably stable, you're more likely to encounter breaking changes when running
+Ansible this way.
+
+If you want to download a tarball of a release, go to
+`releases.ansible.com <https://releases.ansible.com/ansible>`_, though
+most users use ``yum`` (using the EPEL instructions linked above),
+``apt`` (using the PPA instructions linked above), or
+``pip install ansible``.
+
+Design Principles
+=================
+
+*  Have a dead simple setup process and a minimal learning curve
+*  Manage machines very quickly and in parallel
+*  Avoid custom-agents and additional open ports, be agentless by
+   leveraging the existing SSH daemon
+*  Describe infrastructure in a language that is both machine and human
+   friendly
+*  Focus on security and easy auditability/review/rewriting of content
+*  Manage new remote machines instantly, without bootstrapping any
+   software
+*  Allow module development in any dynamic language, not just Python
+*  Be usable as non-root
+*  Be the easiest IT automation system to use, ever.
+
+Get Involved
+============
+
+*  Read `Community
+   Information <https://docs.ansible.com/community.html>`_ for all
+   kinds of ways to contribute to and interact with the project,
+   including mailing list information and how to submit bug reports and
+   code to Ansible.
+*  All code submissions are done through pull requests. Take care to
+   make sure no merge commits are in the submission, and use
+   ``git rebase`` vs ``git merge`` for this reason. If submitting a
+   large code change (other than modules), it's probably a good idea to
+   join ansible-devel and talk about what you would like to do or add
+   first and to avoid duplicate efforts. This not only helps everyone
+   know what's going on, it also helps save time and effort if we decide
+   some changes are needed.
+*  Users list:
+   `ansible-project <https://groups.google.com/group/ansible-project>`_
+*  Development list:
+   `ansible-devel <https://groups.google.com/group/ansible-devel>`_
+*  Announcement list:
+   `ansible-announce <https://groups.google.com/group/ansible-announce>`_
+   -- read only
+*  irc.freenode.net: #ansible
+
+Branch Info
+===========
+
+*  Releases are named after Led Zeppelin songs. (Releases prior to 2.0
+   were named after Van Halen songs.)
+*  The devel branch corresponds to the release actively under
+   development.
+*  Various release-X.Y branches exist for previous releases.
+*  We'd love to have your contributions, read `Community
+   Information <https://docs.ansible.com/community.html>`_ for notes on
+   how to get started.
+
+Authors
+=======
+
+Ansible was created by `Michael DeHaan <https://github.com/mpdehaan>`_
+(michael.dehaan/gmail/com) and has contributions from over 1000 users
+(and growing). Thanks everyone!
+
+Ansible is sponsored by `Ansible, Inc <https://ansible.com>`_
+
+Licence
+=======
+
+GNU General Public License v3.0
+
+See `COPYING <COPYING>`_ to see the full text.
+
+.. |PyPI version| image:: https://img.shields.io/pypi/v/ansible.svg
+   :target: https://pypi.python.org/pypi/ansible
+.. |Build Status| image:: https://api.shippable.com/projects/573f79d02a8192902e20e34b/badge?branch=devel
+   :target: https://app.shippable.com/projects/573f79d02a8192902e20e34b