[documentation] new page on backwards compatibility

[Charlie Lowe]

Laura Scott wrote:
> 
> Good point. I added that as another numeric item in the list.

I think it might not be clear because "version" is ambiguous. To a 
newbie, they might assume 4.7.1 and 4.7.0 count as a current and 
previous release (which they are in one sense). Perhaps something like,

"Only the current stable release series and the previous release series 
(e.g., 4.7 and 4.6) are supported by the Drupal development community."

> 
> . . .only 
> selectively bolded.

Yeah. Bold sort of looses it's effect when overused. How about change 
"Therefore, we offer some fair warning for people considering using 
Drupal:" to a bold title of some kind. And maybe point 6 could be left 
in bold, but not be a list element. See how it sort of sums up the 
previous list items with a concluding bit of advice (the others are not 
advice, but observations about Drupal development).