Add note on Trusted Proxies config when app is behind multiple proxies

I was having the same issue as symfony/symfony#18587

Author: PapyDanone

deployment/proxies.rst

@@ -76,6 +76,12 @@ That's it! It's critical that you prevent traffic from all non-trusted sources.
 If you allow outside traffic, they could "spoof" their true IP address and
 other information.
 
+.. caution::
+
+    In case you are also using `CloudFront`_ or another proxy on top of your Load Balancer, calling ``$request->server->get('REMOTE_ADDR')`` as in the above example won't be enough, as it will only whitelist the first level in the chain of trust (in this case your Load Balancer). You need to make sure you also append the IP addresses or ranges of any additional proxy to the array of trusted proxies. 
+    
+    In the case of `CloudFront`_, you can find the list of IPs `at this address <https://ip-ranges.amazonaws.com/ip-ranges.json>`_. 
+
 Custom Headers When Using a Reverse Proxy
 -----------------------------------------
 
@@ -94,3 +100,4 @@ In this case, you'll need to set the header ``X-Forwarded-Proto`` with the value
 
 .. _`security groups`: http://docs.aws.amazon.com/elasticloadbalancing/latest/classic/elb-security-groups.html
 .. _`CloudFront`: https://en.wikipedia.org/wiki/Amazon_CloudFront
+.. _`at this address`_: https://ip-ranges.amazonaws.com/ip-ranges.json