Adds Caching paper on 2Q algorithm

The algorithm was referenced in an article about
improving buffer cache replacement in OpenBSD.

http://www.tedunangst.com/flak/post/2Q-buffer-cache-algorithm

The article was found on the vldb.org website via a google
search.

http://www.vldb.org/conf/1994/P439.PDF
updates reference to paper
Merge pull request #170 from csaunders/master

Adds Caching paper on 2Q algorithm