[documentation] new page on backwards compatibility

[Charlie Lowe]

Laura Scott wrote:
> 
> Your corrections, improvements, lacerating critiques are invited and 
> welcome. I'm just trying to get a start going here on helping new folks 
> understand the essential philosophy that enables Drupal to be as great 
> as it is.

I like :)

One thing I would mention in the text. If I'm not mistaken, the policy 
is that only the current stable release and the one previous legacy 
version are to be supported? If so, that's a good thing to mention so 
that people get a real feel for how often they need to upgrade. Knowing 
that up front can decrease a lot of frustration later on.

BTW: Knowing this, I often skip one version ;)