Add note to docs for defining custom REGISTER_SERIALIZER

#198

Author: maxim-kht

docs/configuration.rst

@@ -37,6 +37,8 @@ Configuration
     Possible key values:
 
         - REGISTER_SERIALIZER - serializer class in ``rest_auth.register.views.RegisterView``, default value ``rest_auth.registration.serializers.RegisterSerializer``
+    
+        .. note:: The custom REGISTER_SERIALIZER must define a ``def save(self, request)`` method that returns a user model instance
 
 - **REST_AUTH_TOKEN_MODEL** - model class for tokens, default value ``rest_framework.authtoken.models``