chore(node): Add grub-upgrader readme (#8191)

NODE-1712

Author: andrewbattat

ic-os/components/hostos/misc/grub-upgrader/README.md

@@ -0,0 +1,17 @@
+# GRUB Upgrader
+
+`grub-upgrader` propagates GRUB configuration updates from the boot partition 
+to the GRUB partition after IC-OS upgrades.
+
+## How It Works
+
+HostOS and GuestOS upgrade images include the latest `grub.cfg` on the boot partition (`/boot/grub.cfg`),
+but GRUB reads from the GRUB partition (`/boot/grub/grub.cfg`).
+This service syncs them by copying `/boot/grub.cfg` to `/boot/grub/grub.cfg`.
+
+## ⚠️ Backwards Compatibility
+
+If enabling this service, ensure the latest `grub.cfg` is backwards
+compatible with existing GRUB EFI binaries on mainnet.
+This service only updates the config file—not the GRUB EFI binary.
+New syntax unsupported by older GRUB versions may cause boot failures.