Update docs - run python manage.py migrate once

We have to run python manage.py migrate once, in order to create the necessary tables for the postgres search backend.
Otherwise we get an error, as the insert in the table is not possible.

docs/reference/contrib/postgres_search.rst

@@ -63,6 +63,8 @@ Give it the alias `'default'` if you want it to be the default search backend:
         },
     }
 
+After installing the module, run ``python manage.py migrate`` to create the necessary ``postgres_search_indexentry`` table.
+
 You then need to index data inside this backend using
 the :ref:`update_index` command. You can reuse this command whenever
 you want. However, it should not be needed after a first usage since