add explanation to required slots  RasaHQ/roadmap#280
@@ -33,7 +33,11 @@ class FormAction(Action):
@staticmethod
def required_slots(tracker):
# type: (Tracker) -> List[Text]
- """A list of required slots that the form has to fill"""
+ """A list of required slots that the form has to fill
+
+ Use `tracker` to request different list of slots
+ depending on the state of the dialogue
+ """
raise NotImplementedError("A form must implement required slots "
"that it has to fill")
