Add help texts to ModelSerializer property fields

If a field of a model wich is a read-only property is included in a ModelSerializer, automátically set its help text to the docstring of the property function. I used this to work with the documentation tool django-rest-swagger.

rest_framework/serializers.py

@@ -1160,7 +1160,7 @@ class ModelSerializer(Serializer):
         Create a read only field for model methods and properties.
         """
         field_class = ReadOnlyField
-        field_kwargs = {}
+        kwargs = {'help_text': getattr(model, field_name).__doc__}
 
         return field_class, field_kwargs