fixes #5692: add version # to readme

README.md

-[Twitter Bootstrap](http://twitter.github.com/bootstrap) [![Build Status](https://secure.travis-ci.org/twitter/bootstrap.png)](http://travis-ci.org/twitter/bootstrap)
+[Twitter Bootstrap v2.2.1](http://twitter.github.com/bootstrap) [![Build Status](https://secure.travis-ci.org/twitter/bootstrap.png)](http://travis-ci.org/twitter/bootstrap)
 =================
 
 Bootstrap is a sleek, intuitive, and powerful front-end framework for faster and easier web development, created and maintained by [Mark Otto](http://twitter.com/mdo) and [Jacob Thornton](http://twitter.com/fat).