added note about Redis to performance section

docs/guide/performance.md

@@ -100,6 +100,8 @@ You can use `CacheSession` to store sessions using cache. Note that some
 cache storage such as memcached has no guarantee that session data will not
 be lost leading to unexpected logouts.
 
+If you have [Redis](http://redis.io/) on your server, it's highly recommended as session storage.
+
 Improving application
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