Removes most of the content from the README to avoid it becoming outdated
and stale given that our currently maintained docs are now on
http://docs.openstack.org .  In some cases, including keystone config,
extension writing, etc. this means text from the README is being
transferred directly from the README to an external doc so we make
sure we don't lose anything.

Change-Id: Ie08db4bd4854bb45e6777b1e0abe37f51d3e5c5c