[Review] Added detailed Backwards Compatibility Promise text

I created a detailed text about which parts of the Symfony code are covered by our BC promise. The goals are to:

* tell our users which parts exactly are safe to use
* tell our contributors which changes they are allowed to do

I'll continue working on the individual rules of allowed changes. Nevertheless, let me hear your feedback.

##### Update (2014/01/10)

The draft is ready for review now.