Link to FOSHttpCacheBundle

* Link to FOSHttpCacheBundle that can help with cache invalidation.
* Qualify the strong negative opinion about cache invalidation, which we also find in the [Cache Invalidation section](http://symfony.com/doc/current/book/http_cache.html#http-cache-invalidation) in the docs. The cookbook (and docs) are too opiniated on this subject and offer no balanced view. We should not forget that [RFC 2616](http://tools.ietf.org/html/rfc2616#section-13.10) considers invalidation seriously. I would love to hear from the original  authors of this doc section or any others what their arguments against invalidation are. Let’s try to introduce both benefits and drawbacks more carefully in the docs (e.g., as is done [here](http://foshttpcache.readthedocs.org/en/latest/invalidation-introduction.html)).

When we link to a Symfony bundle, should we link to its official home page (`https://github.com/FriendsOfSymfony/FOSHttpCacheBundle` in this case) or to its official documentation (`http://foshttpcachebundle.readthedocs.org/` in this case)?

I don't know if there is an official policy set by the documentation maintainers: @weaverryan @WouterJ @xabbuh