Document adding django_filters to installed apps

Reminds users to add `django_filters` to their `INSTALLED_APPS` as detailed in the django-filter documentation https://django-filter.readthedocs.io/en/develop/guide/install.html

docs/api-guide/filtering.md

@@ -142,7 +142,7 @@ Note that you can use both an overridden `.get_queryset()` and generic filtering
 The `django-filter` library includes a `DjangoFilterBackend` class which
 supports highly customizable field filtering for REST framework.
 
-To use `DjangoFilterBackend`, first install `django-filter`.
+To use `DjangoFilterBackend`, first install `django-filter`. Then add `django_filters` to Django's `INSTALLED_APPS`
 
     pip install django-filter