read prefs: adding warning about primary prefered and older mongos instances.

Author: Sam Kleinman

source/applications/replication.txt

@@ -251,6 +251,15 @@ behavior <replica-set-read-preference-behavior>`. See also the
    operations using the :readmode:`primaryPreferred` mode may return
    stale data in some situations.
 
+   .. warning::
+
+      .. versionchanged:: 2.2
+         :program:`mongos` added full support for read preferences.
+
+      When connecting to a :program:`mongos` instance older 2.2, using
+      a client that supports read preference modes,
+      :readmode:`primaryPrefered` will send queries to secondaries.
+
 .. readmode:: secondary
 
    Operations read *only* from the :term:`secondary` members of the set.