docs: Add kernel version packaging recommendations

Signed-off-by: Jens Reidel <adrian@postmarketos.org>
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7295>

docs/index.md

@@ -15,5 +15,6 @@ in pmaports and the processes around device categorization.
 
    approval-rules
    device-categorization
+   kernel-versions
 ```
 

docs/kernel-versions.md

@@ -0,0 +1,31 @@
+# Kernel Versions
+
+Generally speaking, maintainers have the freedom to package any kernel version
+they desire for their devices. However, the choice affects the categories that
+the device may be eligible for and we strongly recommend following some
+additional guidelines for which kernel version to package for a device.
+
+## Categorization
+
+Devices with a close-to-mainline kernel usually reside in `testing` and can be
+moved to `community` or `main` later if they meet the necessary
+[requirements](./device-categorization.md). Devices using a downstream (i.e.
+vendor-provided) kernel are packaged in the `downstream` or `archived`
+categories.
+
+## What To Package
+
+We strongly advise **against** using any
+[linux-next](https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git)
+version as the kernel packaged for devices. linux-next is used for integration
+testing and as a base for patch submission, but is by no means stable and
+therefore we discourage packaging it. One exception to this is the
+`device-postmarketos-trailblazer` device package - it is intended to be a
+bleeding edge target reflecting the very latest state of upstream.
+
+Further, we do not recommend packaging the first release candidate (-rc1)
+tagged after the merge window is closed. The first release candidate introduces
+a lot of new changes, after which only fixes are admitted into the kernel tree.
+Therefore, subsequent release candidates tend to be more stable than the first.
+We recommend waiting until at least -rc2 before packaging a new major kernel
+release.