Change in coreboot[master]: Documentation: Add list of boards maintained on each branch

[Martin L Roth (Code Review)]

Attention is currently required from: Paul Menzel, Angel Pons, Arthur Heymans, Werner Zeh.
Martin L Roth has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/63797 )

Change subject: Documentation: Add list of boards maintained on each branch
......................................................................

Patch Set 2:

(2 comments)

File Documentation/releases/boards_supported_on_branches.md:

https://review.coreboot.org/c/coreboot/+/63797/comment/b9acff53_6b5ace88 
PS2, Line 8: release
> or commit

Done

https://review.coreboot.org/c/coreboot/+/63797/comment/9e0bf19d_635b98ca 
PS2, Line 17: can also be created on request.
> Maybe it should also be mentioned, that commits can be backported to these branches?

Done