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.

 [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

README.md

@@ -50,6 +50,20 @@ Issues are being tracked
 * ps
 * tar
 
+## Relation to pmaports
+For pmbootstrap to be useful, it needs to maintain a local copy of the
+[pmaports](https://gitlab.com/postmarketOS/pmaports) repository where
+postmarketOS-specific packages are maintained. This is set up automatically, but
+the local copy of pmaports does not automatically get updated. To update it, you
+can run `$ pmbootstrap pull`.
+
+Additionally, due to the mutual dependence and lack of a stable API between
+pmbootstrap and pmaports, we currently only guarantee that pmbootstrap works
+with [active releases](https://wiki.postmarketos.org/wiki/Releases). Attempting
+to work with old releases with recent pmbootstrap versions may result in
+failures and is not supported. Currently there is no documentation for which
+releases a given pmbootstrap version supports.
+
 ## Usage Examples
 Please refer to the [postmarketOS wiki](https://wiki.postmarketos.org) for
 in-depth coverage of topics such as