[placement] Add api-ref for RP usages

Add placement-api-ref for resource provider usages.

Co-Authored-by: Andrey Volkov <avolkov@gmail.com>

Change-Id: I6ba8765a427f654dafb32fc3fbf5492b0e1b426a

placement-api-ref/source/index.rst

@@ -4,8 +4,6 @@
  Placement API
 ===============
 
-.. TODO(cdent) this is a big pile of todo
-
 This is a reference for the Openstack Placement API. To learn more about
 Openstack Placement API concepts, please refer to the
 `Placement Introduction <http://docs.openstack.org/developer/nova/placement.html>`_.
@@ -25,3 +23,4 @@ Openstack Placement API concepts, please refer to the
 .. include:: allocations.inc
 .. include:: resource_provider_allocations.inc
 .. include:: usages.inc
+.. include:: resource_provider_usages.inc