Revamp README file
Revamp README file

The README used only python-*client objects, thus defeating the
purpose of shade, which is about putting a facade for all those
interfaces to simplify things.
README now shows you can use shade simplified interface and also
access the underlying python-*client API objects.

Change-Id: I0c638beab615177fc6ce3704c5443a3394a87ed0

openstack/python-openstacksdk,stackforge/python-openstacksdk,jsmartin/shade,stackforge/python-openstacksdk,openstack-infra/shade,dtroyer/python-openstacksdk,openstack-infra/shade,jsmartin/shade,dtroyer/python-openstacksdk,openstack/python-openstacksdk