Add note about chord support in older versions of Redis

They simply don't work in versions before 2.2.

Mailing list reference:
https://groups.google.com/d/msg/celery-users/aEeEgJhJ5Ys/lGbUZw4OlW0J

Signed-off-by: Dan McGee <dan@archlinux.org>

docs/userguide/tasksets.rst

@@ -246,10 +246,12 @@ Example implementation:
 
 This is used by all result backends except Redis, which increments a
 counter after each task in the header, then applying the callback when the
-counter exceeds the number of tasks in the set.
+counter exceeds the number of tasks in the set. *Note:* chords do not properly
+work with Redis before version 2.2; you will need to upgrade to at least 2.2 to
+use them.
 
 The Redis approach is a much better solution, but not easily implemented
-in other backends (suggestions welcome!)
+in other backends (suggestions welcome!).
 
 
 .. note::