Adds multi-backend support documentation

This patch adds multi-backend support documentation for cinder.

This patch was made with the help of:
https://wiki.openstack.org/wiki/Cinder-multi-backend

Link to the patch:
https://review.openstack.org/#/c/21815

This patch fixes the bug 1155622

Change-Id: If47752782e1b690845d1f393a78f5c9a8e4b9171