'UserFilter' in readme example is not defined or imported anywhere

[flyte]

In the line from the readme:

    users3 = DjangoListObjectField(UserListType, filterset_class=UserFilter, description='All Users query')

you reference UserFilter but that isn't imported or created anywhere in the example.

[apelliciari]

just a quick tip: look here for the definition

https://github.com/eamigo86/graphene-django-extras/blob/abb5a333ef74a3c0aa0ce50ee896ae1c3c3b3c32/tests/filtersets.py