[placement] Add api-ref for usages

Change-Id: I50954991f80725dc22be95eeecded2ee1e005e97

placement-api-ref/source/get-usages.json

@@ -0,0 +1,7 @@
+{
+    "usages": {
+        "DISK_GB": 5,
+        "MEMORY_MB": 512,
+        "VCPU": 2
+    }
+}

placement-api-ref/source/index.rst

@@ -24,3 +24,4 @@ Openstack Placement API concepts, please refer to the
 .. include:: resource_provider_traits.inc
 .. include:: allocations.inc
 .. include:: resource_provider_allocations.inc
+.. include:: usages.inc

placement-api-ref/source/usages.inc

@@ -0,0 +1,43 @@
+======
+Usages
+======
+
+Represent the consumption of resources for a project and user.
+
+.. note:: Usages API requests are available starting from version 1.9.
+
+List usages
+===========
+
+Return a report of usage information for resources associated with the
+project identified by `project_id` and user identified by
+`user_id`. The value is a dictionary of resource classes paired with
+the sum of the allocations of that resource class for provided
+parameters.
+
+.. rest_method:: GET /usages
+
+Normal Response Codes: 200
+
+Error response codes: badRequest(400)
+
+Request
+-------
+
+.. rest_parameters:: parameters.yaml
+
+  - project_id: project_id
+  - user_id: user_id
+
+Response
+--------
+
+.. rest_parameters:: parameters.yaml
+
+  - usages: resources
+
+Response Example
+----------------
+
+.. literalinclude:: get-usages.json
+   :language: javascript