Documenting how we should use the versionadded tag - see https://groups....

Hi guys!

This documents the `versionadded` tag that Fabien created that we've been using for awhile. There's a discussion on the dev board as to whether or not we should stop using this or formally document it. This is a PR for documenting how to properly use it.

The rest of the conversation is here:  https://groups.google.com/forum/#!msg/symfony-devs/-6HAQgAB2LY/uBSx8uYJQdQJ

Thanks!