feature #4175 Link to FOSHttpCacheBundle (ddeboer)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4175).

Discussion
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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)).

Commits
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9243612 Link to FOSHttpCacheBundle

Author: weaverryan

cookbook/cache/varnish.rst

@@ -91,10 +91,18 @@ Symfony2 adds automatically:
 Cache Invalidation
 ------------------
 
-You should never need to invalidate cached data because invalidation is already
-taken into account natively in the HTTP cache models (see :ref:`http-cache-invalidation`).
+If you want to cache content that changes frequently and still serve 
+the most recent version to users, you need to invalidate that content. 
+While `cache invalidation`_ allows you to purge content from your 
+proxy before it has expired, it adds complexity to your caching setup.
 
-Still, Varnish can be configured to accept a special HTTP ``PURGE`` method
+.. tip::
+
+    The open source `FOSHttpCacheBundle`_ takes the pain out of cache 
+    invalidation by helping you to organize your caching and 
+    invalidation setup. 
+
+Varnish can be configured to accept a special HTTP ``PURGE`` method
 that will invalidate the cache for a given resource:
 
 .. code-block:: text
@@ -232,3 +240,5 @@ absolute URLs:
 .. _`Edge Architecture`: http://www.w3.org/TR/edge-arch
 .. _`GZIP and Varnish`: https://www.varnish-cache.org/docs/3.0/phk/gzip.html
 .. _`Surrogate-Capability Header`: http://www.w3.org/TR/edge-arch
+.. _`cache invalidation`: http://tools.ietf.org/html/rfc2616#section-13.10
+.. _`FOSHttpCacheBundle`: http://foshttpcachebundle.readthedocs.org/