Add plug-in summary for osc doc

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http://docs.openstack.org/developer/python-openstackclient/plugin-commands.html

Change-Id: I466e71dcbe03ca41db0eb9593911d9b4f3591d53

mistralclient/osc/plugin.py

@@ -11,6 +11,8 @@
 #    See the License for the specific language governing permissions and
 #    limitations under the License.
 
+"""OpenStackClient plugin for Workflow service."""
+
 import logging
 
 from osc_lib import utils