Add plug-in summary for osc doc

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

Change-Id: I4958ec1dfda96f54b75963a04b5a4fc9eec09d84

heatclient/osc/plugin.py

@@ -11,6 +11,8 @@
 #   under the License.
 #
 
+"""OpenStackClient plugin for Orchestration service."""
+
 import logging
 
 from osc_lib import utils