Doc: add a note about the URL namespace for pluggable URLs

EXTENDING.md

@@ -169,3 +169,5 @@ Optionaly, you can customize which URLs are plugged and to which prefix via the
         class MyAppConfig(AppConfig, coin.apps.AppURLS):
             name = 'my_app'
             exported_urlpatterns = [('coolapp', 'my_app.cool_urls')]
+
+The URL namespace will be set to the prefix ('coolapp' in this example).