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
minor #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 #3778).

Discussion
----------

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
-------

8f0f89d Adding information on internal reverse proxy