Add link to library

Author: ddeboer

README.md

@@ -11,7 +11,8 @@ Introduction
 This bundle offers tools to improve HTTP caching with Symfony2. It provides
 global configuration options to set caching headers based on the path,
 controller and other aspects of the request. In addition, it provides services
-for the FOSHttpCache library tools to actively invalidate caching proxies and
+for the [FOSHttpCache library](https://github.com/FriendsOfSymfony/FOSHttpCache) 
+tools to actively invalidate caching proxies and
 some additional tools that can help when working with a caching proxy.
 
 Features