Updated README file.

The new instructions detail 2 kinds of installs:
with devstack and without.

The devstack instructions were copied from contrib/devstack/README.rst
to ensure users viewing congress/README.rst would have the full
instructions.  Having to keep two copies of the devstack install
instructions in sync is unfortunate, but worth the benefit.

Change-Id: I853ab2ef76fe1f6a7ebdeec93fc1864131b9488b