Bring in optional runas user for enabling plugins

Change-Id: I4410b73b497ff35fd5afc309a5372f152cdd4f9d
diff --git a/README.rst b/README.rst
index 16f89e8..bab7652 100644
--- a/README.rst
+++ b/README.rst
@@ -49,6 +49,7 @@
             enabled: true
             user: 'monitor'
             password: 'password'
+        plugins_runas_user: rabbitmq
         plugins:
         - rabbitmq_stomp