add note about cache warming

Author: kbond

cookbook/templating/namespaced_paths.rst

@@ -74,6 +74,10 @@ directory:
             );
         ));
 
+.. caution::
+
+    These paths are not warmed during cache warming. This is fixed in 2.8.
+
 The registered namespace is called ``foo_bar``, which refers to the
 ``vendor/acme/foo-bar/templates`` directory. Assuming there's a file
 called ``sidebar.twig`` in that directory, you can use it easily: