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!
I wonder if we could add a brief paragraph explaining the basics of *semver* for those unaware of it. Something like this:

```
Symfony project uses `Semantic Versioning`_ (or *semver* for short) to release its new versions.
This means that Symfony versions are composed of three numbers (``X.Y.X``) with the following
meaning:

* ``X`` is called the **major version** and it's increased when Symfony makes
  incompatible API changes,
* ``Y`` is called the **minor version** and it's increased when Symfony adds new
  features in a backwards-compatible manner,
* ``Z`` is called the **patch version** and it's increased when Symfony makes
  backwards-compatible bug fixes.

In other words, changing the patch or the minor version numbers should be *safe* for
your application, but changing the major version number would require some work to
update the code of your application.
```