diff --git a/docs/source/design.rst b/docs/source/design.rst
index 12ded84e..f606f43b 100644
--- a/docs/source/design.rst
+++ b/docs/source/design.rst
@@ -123,6 +123,9 @@ as the application code will be shared among workers but loaded only in the
 worker processes (unlike when using the preload setting, which loads the code in
 the master process).
 
+.. note:: Under Python 2.x, uou need to install the 'futures' package to use
+this feature.
+
 .. _Greenlets: https://github.com/python-greenlet/greenlet
 .. _Eventlet: http://eventlet.net
 .. _Gevent: http://gevent.org