Adding a new entry about reverse proxies in the framework

Hi guys!

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all (or 2.3+)
| Fixed tickets | #2946 #2491

Per #2491, I wanted to answer the simple question: "What do I need to do in order to configure Symfony if I have a reverse proxy". The `trusted_proxies` is already documented in the reference section, but this is a full walk-through of what you should be doing and why. I've also increased links in several places where notes are needed.

Thanks!
standards say that there should be an empty line before this line