[documentation] new page on backwards compatibility

[Charlie Lowe]

Laura Scott wrote:
> 
> Are we starting to get almost close?
> 

I would say yes :)

The only other thing I can think of is cross-linking; this is something 
we don't do enough of when creating new pages.

For instance, the version numbers page 
(http://drupal.org/handbook/version-info) should link to this one. I'm 
not as familiar with the content in the Developing for Drupal book 
(http://drupal.org/contributors-guide), but it ought to be linked to in 
there somewhere, too, to make sure that developers see it.