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.

README.md

@@ -50,6 +50,22 @@ 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`.
+
+The latest pmbootstrap version works with currently
+[active postmarketOS releases](https://wiki.postmarketos.org/wiki/Releases).
+Attempting to use pmboostrap with old postmarketOS versions (old pmaports
+branches) may result in failures and is not supported. See
+`pmbootstrap_min_version` in
+[pmaports.cfg](https://wiki.postmarketos.org/wiki/Pmaports.cfg_reference) for
+the oldest supported pmbootstrap version for a given pmaports revision. The
+upper bound is not documented.
+
 ## Usage Examples
 Please refer to the [postmarketOS wiki](https://wiki.postmarketos.org) for
 in-depth coverage of topics such as