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)).

Afaik there is no strict guideline regarding this. But linking to the official documentation seems reasonable to me since this is what people are usually interested in when reading the documentation. We do this in the [Serializer Component documentation](http://symfony.com/doc/current/components/serializer.html#jmsserializer) for the JMS Serializer for example (the CMF is linking to https://github.com/schmittjoh/JMSSerializerBundle btw., @WouterJ).