README.md: Add section about pmbootstrap's relation to pmaports (MR 2421)

This is section is not meant to represent my opinions, rather I seek to
document the status quo. I base the support range on what's been
reasoned in prior merge requests such as [1] and [2], as well as [3]
which only fixed CI in the one active stable release and left old stable
releases unfixed.

While I don't necessary think this is a great situation given that old
releases may sometimes be useful, extending the time pmbootstrap has to
support old releases would incur significant additional maintenance
effort and consequently require more discussion than just documenting
what's already happening. As such, I think this is the most reasonable
thing to do for now.

 [1]: https://gitlab.com/postmarketOS/pmbootstrap/-/merge_requests/2360#note_1999012908
 [2]: https://gitlab.com/postmarketOS/pmbootstrap/-/merge_requests/2417#note_2135527528
 [3]: https://gitlab.com/postmarketOS/pmaports/-/merge_requests/5676