add note to zamboni-login waffle switch

migrations/292-login-waffle.sql

@@ -1 +1,5 @@
-INSERT INTO waffle_switch (name, active) VALUES ('zamboni-login', 0);
+INSERT INTO waffle_switch (name, active, note)
+    VALUES ('zamboni-login', 0,
+            'Flip this when AMO ditches remora and uses django for '
+             'authentication. Enabling this switch will show success messages '
+             'when users log in and register via Django.');