Created Documentation CHANGELOG

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes
| Applies to | 2.3+
| Fixed tickets | #2579

This is the second enhancement (after #3328) of the Symfony2 Documentation Process.

This idea came from @shoomyth in #2579. Since @weaverryan is using gh to merge the Pull Requests, it's easy to scan through the commits and find the changes (I'm busy creating an automated script for this).

The plan is to update the changelog each month, so people have a clear view on new articles or fixed things. It'll also be another attribution to the many symfony doc hero's!
This PR lists the changes in the 2.3 branch for January. When we agree with this, it'll be update for changes in the 2.4 and master branches.

My idea was to put a link to this article in the sidebar (ping @fabpot).

We would love to get your opinion about this new change!