Add an introduction to the documentation
Add an introduction to the documentation

This adds a global introduction to the documentation, explaining what
CloudKitty is and does.

Change-Id: I4aa65d63970e4a031bf662f42e2fffedf1627f41
Story: 2004179
Task: 28153

stackforge/cloudkitty,openstack/cloudkitty,openstack/cloudkitty,stackforge/cloudkitty