Add reminder to deprecate plugin spec

flaskbb/plugins/spec.py

@@ -197,6 +197,7 @@ def flaskbb_event_topic_save_after(topic, is_new):
     """
 
 
+# TODO(anr): When pluggy 1.0 is released, mark this spec deprecated
 @spec
 def flaskbb_event_user_registered(username):
     """Hook for handling events after a user is registered