[AIRFLOW-XXX] Add note about plugin docs (#4706)

docs/plugins.rst

@@ -97,6 +97,14 @@ looks like:
         appbuilder_views = []
         # A list of dictionaries containing FlaskAppBuilder BaseView object and some metadata. See example below
         appbuilder_menu_items = []
+        # A callback to perform actions when airflow starts and the plugin is loaded.
+        # NOTE: Ensure your plugin has *args, and **kwargs in the method definition
+        #   to protect against extra parameters injected into the on_load(...)
+        #   function in future changes
+        def on_load(*args, **kwargs):
+           # ... perform Plugin boot actions
+           pass
+