Added notification concerning the migrations

This should prevent others from running into #405 while trying to create both a swappable instance and swap it out at once

Author: bastbnl

docs/advanced_topics.rst

@@ -43,6 +43,10 @@ Write something like this in your settings module::
 
     OAUTH2_PROVIDER_APPLICATION_MODEL='your_app_name.MyApplication'
 
+Be aware that, when you intend to swap the application model, you should create and run the 
+migration defining the swapped application model prior to setting OAUTH2_PROVIDER_APPLICATION_MODEL. 
+You'll run into models.E022 in Core system checks if you don't get the order right.
+
 That's all, now Django OAuth Toolkit will use your model wherever an Application instance is needed.
 
     **Notice:** `OAUTH2_PROVIDER_APPLICATION_MODEL` is the only setting variable that is not namespaced, this