Add minimal documentation to ESP sync hook

Author: zdykstra

contrib/esp-sync.sh

@@ -1,5 +1,21 @@
 #!/bin/bash
 
+## This script can be used by generate-zbm to keep two or more ESPs in sync 
+## after an EFI is built. Place the script in the directory defined by:
+##
+##   Global.PostHooksDir
+##
+## Ensure the file is executable. Adjust the ESPS variable to include
+## the correct disks / partitions for your BACKUP EFI partitions. Do NOT include
+## the primary/master ESP. Your master EFI mountpoint will be read, and used
+## as the base for copying files to your other partitions.
+## 
+## Only files with `zfsbootmenu` in the name are copied to / deleted from
+## the backup ESPs. It will not manage any other files for you.
+## 
+## This hook relies on yq-go and rsync.
+
+
 cleanup() {
   if [ -n "${ESP_MNT}" ]; then
     mountpoint -q "${ESP_MNT}" && umount -R "${ESP_MNT}"