fixed settings documentation

Author: masci

docs/settings.rst

@@ -1,7 +1,9 @@
 Settings
 ========
 
-Our configurations are all namespaced under the `OAUTH2_PROVIDER` settings (yes, like Django REST Framework ;) thanks guys!).
+Our configurations are all namespaced under the `OAUTH2_PROVIDER` settings with the solely exception of
+`OAUTH2_PROVIDER_APPLICATION_MODEL`: this is because of the way Django currently implements
+swappable models. See issue #90 (https://github.com/evonove/django-oauth-toolkit/issues/90) for details.
 
 For example:
 
@@ -19,3 +21,5 @@ For example:
 
 
 TODO: add reference documentation for DOT settings
+
+A big *thank you* to the guys from Django REST Framework for inspiring this.