キャッシュに関する記事を追加

Squashed commit of the following:

commit dc062ba194c6737841e8d8b14a4a53269b1bab6b
Author: Yuu ISHIKAWA <yuu.ishikawa@gmail.com>
Date:   Mon Mar 31 15:57:52 2014 +0900

    update files

commit b15ada59f4763e66e2693f895c5dfd0376ecb1af
Author: Yuu ISHIKAWA <yuu.ishikawa@gmail.com>
Date:   Mon Mar 31 15:20:32 2014 +0900

    add documents for cache