[documentation] new page on backwards compatibility

[Laura Scott]

Revised:

> 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."
>

Revised sentence as suggested. Page further revised for clarity about  
major releases.

Added link to Steven's page on versions (http://drupal.org/handbook/ 
version-info), for reference.

>> . . .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).

Boldface reduced, with a caveat emptor header above ordered list.

Are we starting to get almost close?

Laura