Adding a guide about upgrading

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.6+
| Fixed tickets | #4172 

Hi guys!

First, the plan is to merge this into 2.6, then backport it to 2.3 (just remove the 2.5 and 2.6 sections about upgrading).

I put this together fairly quickly, and I would really like comments. I've been in front of the computer for awhile, so I'm not convinced this is quite as smooth yet as I'd like it to be!

Overall, I want to fix the issue of "How to I upgrade". We'll say - just go to this page, and it'll walk you through the steps, and even reassure you that you don't need to make any changes to your code (or, you only need to make these very specific changes).

Thanks!
As a way to introduce Composer versions used in the following sections, maybe we should mention that Symfony by default uses `2.6.*` as its version constraint, and that's why you can execute that command to update the patch version.