Added a note about postgis_topology in install docs

Thanks Paolo Corti for the suggestion.

docs/ref/contrib/gis/install.txt

@@ -523,6 +523,9 @@ spatial functionality::
     > CREATE EXTENSION postgis;
     > CREATE EXTENSION postgis_topology;
 
+No PostGIS topology functionalities are yet available from GeoDjango, so the
+creation of the ``postgis_topology`` extension is entirely optional.
+
 .. _spatialdb_template_earlier:
 
 Creating a spatial database template for earlier versions