docs: Add instructions to enable 'site' framework we depend on.

@@ -6,6 +6,10 @@ To get started using the ``comments`` app, follow these steps:
 
 #. Install the comments app by running ``pip install django-contrib-comments``.
 
+#. :ref:`Enable the "sites" framework <enabling-the-sites-framework>` by adding
+   ``'django.contrib.sites'`` to :setting:`INSTALLED_APPS` and defining
+   :setting:`SITE_ID`.
+
 #. Install the comments framework by adding ``'django_comments'`` to
    :setting:`INSTALLED_APPS`.