minor symfony#3778 Adding information on internal reverse proxy (tcz)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes symfony#3778).

Discussion
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Adding information on internal reverse proxy

For more information see: http://stackoverflow.com/questions/9029757/how-do-i-get-the-user-ip-address-in-symfony2-controller

Commits
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8f0f89d Adding information on internal reverse proxy

components/http_foundation/trusting_proxies.rst

@@ -24,6 +24,12 @@ your proxy.
 
     // only trust proxy headers coming from this IP addresses
     Request::setTrustedProxies(array('192.0.0.1', '10.0.0.0/8'));
+    
+.. note::
+
+   When using Symfony's internal reverse proxy (``AppCache.php``) make sure to add 
+   ``127.0.0.1`` to the list of trusted proxies.
+
 
 Configuring Header Names
 ------------------------