Is not clear in README which versions of Graphene Django supports

Great working supporting this. It has been really helpful.

I am using in my stack:
```
graphene==2.1.5
graphene-django==2.3.0
django-graphql-geojson==0.1.4
```

So far it works fine but I want to upgrade graphene-django and graphene and I am not sure whether django-graphql-geojson will support it. Would you mind telling me up to which version supports graphene-django?