Adds instruction for including in urlconf

I received an error `Reverse for 'django_select2_central_json' with arguments '()' and keyword arguments '{}' not found.`, and found an answer [here](Reverse for 'django_select2_central_json' with arguments '()' and keyword arguments '{}' not found.)

Presumably everybody needs to include this in the urlconf, as I'm just using the basic fields you've provided.

Nice work on this package!