Mention keystoneclient.Session use in docs

Jamie added some excellent "Using Sessions" docs to keystoneclient in
I5e44c1029ce160cb2798cfb8a535aa9f3311799a. These will be published to

  http://docs.openstack.org/developer/python-keystoneclient/using-sessions.html

once the version after 0.9.0 is released.

Let's add a brief example on how to use this API and reference the
keystoneclient docs.

Change-Id: Icbcef45f13c1f962c90aa3db9dde4360520166ff
Merge "Mention keystoneclient.Session use in docs"