README.md: Add section about pmbootstrap's relation to pmaports

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.