Update CHANGELOG.

CHANGELOG

@@ -16,9 +16,9 @@ v0.6.6 (2011-xx-xx)
   * ZUNIONSTORE and ZINTERSTORE can accept an array to specify a list of the 
     source keys to be used to populate the destination key.
 
-  * MGET, SINTER and SUNION can accept an array to specify the list of keys. 
-    SINTERSTORE and SUNIONSTORE can also accept an array to specify the list 
-    of source keys.
+  * MGET, SINTER, SUNION and SDIFF can accept an array to specify the list of 
+    keys. SINTERSTORE, SUNIONSTORE and SDIFFSTORE can also accept an array to 
+    specify the list of source keys.
 
   * SUBSCRIBE and PSUBSCRIBE can accept a list of channels for subscription.