docs/generic-kernels: Add device package template

This is currently hidden away in a GitLab issue comment. For broader
visibility, let's just put this in the documentation to encourage people
to follow it.

Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8108>

docs/generic-kernels.md

@@ -80,3 +80,35 @@ The following types of patches can be temporarily added to the generic kernels:
   get the patch fixed or reverted).
 
 Out-of-tree patches are not acceptable for the generic kernels.
+
+## Device package template
+
+For a consistent packaging setup, we recommend following this template when
+using the generic kernels in a device package:
+
+```shell
+subpackages="
+  $pkgname-kernel-stable:kernel_stable
+  $pkgname-kernel-lts:kernel_lts
+  $pkgname-kernel-mainline:kernel_mainline
+  "
+...
+
+kernel_stable() {
+  pkgdesc="Stable kernel (recommended, best balance between stability and features)"
+  depends="linux-postmarketos-stable"
+  devicepkg_subpackage_kernel $startdir $pkgname $subpkgname
+}
+
+kernel_lts() {
+  pkgdesc="Long-term maintainance kernel (most stability, not all security fixes & new features)"
+  depends="linux-postmarketos-lts"
+  devicepkg_subpackage_kernel $startdir $pkgname $subpkgname
+}
+
+kernel_mainline() {
+  pkgdesc="Upstream development kernel (regular breakage, latest features)"
+  depends="linux-postmarketos-mainline"
+  devicepkg_subpackage_kernel $startdir $pkgname $subpkgname
+}
+```