added a tip about debugging caching problems (closes #135)

doc/providers/http_cache.rst

@@ -82,5 +82,17 @@ The provider also provides ESI support::
 
     $app['http_cache']->run();
 
+.. tip::
+
+    To help you debug caching issues, set your application ``debug`` to true.
+    Symfony automatically adds a ``X-Symfony-Cache`` header to each response
+    with useful information about cache hits and misses.
+
+    If you are *not* using the Symfony Session provider, you might want to set
+    the PHP ``session.cache_limiter`` setting to an empty value to avoid the
+    default PHP behavior.
+
+    Finally, check that your Web server does not override your caching strategy.
+
 For more information, consult the `Symfony2 HTTP Cache documentation
 <http://symfony.com/doc/current/book/http_cache.html>`_.