UKI: document where UKI addons provided via package should be

According with
systemd/systemd#29372 (comment)

specs/unified_kernel_image.md

@@ -159,3 +159,22 @@ that tools staging or consuming UKIs have a common place to store and look for t
 
 The installed UKIs should have a filename `<version format specification>.efi`, i.e. the filename is left to
 implementers but must be valid for comparisons according to the [Version Format Specification](version_format_specification.md).
+
+UKI addons (cmdline PE addons, systemd-sysext extensions for the initrd) built centrally by distributions and
+installed via package manager should be installed into locations depending on whether they are global UKI addons,
+that should be applied to all UKIs installed in the ESP, or whether they should only be applied to a
+single specific UKI.
+
+Global UKI addons should be installed into `/usr/lib/modules/uki.extra.d/`. UKI-specific addons should be installed
+to `/usr/lib/modules/$UNAME/$UKI.efi.extra.d/`, where `$UNAME` is the output of `uname -r` of the kernel included
+in the UKI and `$UKI` is the name of the corresponding centrally built UKI with the `.efi` extension stripped.
+
+The installed UKI addons should have a filename `<version format specification>.addon.efi`, i.e. the filename
+is left to implementers but must be valid for comparisons according to the
+[Version Format Specification](version_format_specification.md).
+
+### Example
+Given `UNAME="6.9.1-1-foo"` and `UKI="bar.efi"`:
+* UKI will be in `/usr/lib/modules/6.9.1-1-foo/bar.efi`
+* Global addons will be in `/usr/lib/modules/uki.extra.d/`
+* Local addons will be in `/usr/lib/modules/6.9.1-1-foo/bar.efi.extra.d/`