Add filterset_fields lookup expresson to docs (#1460)

Author: jvacek

docs/guide/tips.txt

@@ -37,6 +37,16 @@ would be:
 
     class ProductFilter(django_filters.FilterSet):
         price__gt = django_filters.NumberFilter(field_name='price', lookup_expr='gt')
+        
+When using ``filterset_fields``, you can also add the ``lookup_expr`` in the
+dict of fields like so: 
+
+.. code-block:: python
+    # ... ModelViewSet with DjangoFilterBackend in filter_backends ...
+    
+    filterset_fields = {
+        "price": ["gt", "exact"],
+    }
 
 
 Missing ``lookup_expr`` for text search filters