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!