Document example that uses factory_boy with Django GenericForeignKeys

I created factories today for models with Django GenericForeignKeys. It was possible to do but not entirely straightforward. I think anyone else who want is making a factory for a model with a GenericForiegnKey would appreciate some documentation of how to create a factory for such a relationship. This pull requests adds an example that shows how to do it.

I'm not sure if the why that did it is the best why. If there is a better why I would appreciate advice/documentation on how to do it.