Update i_u_m_update.md

docs/i_u_m_update.md

@@ -22,6 +22,9 @@ Some minor conflicts will be auto-corrected (in favour for the mailcow: dockeriz
 # Update with merge strategy "ours" instead of "theirs" 
 # This will merge in favor for your local changes.
 ./update.sh --ours
+
+# Don't update, but prefetch images and exit
+./update.sh --prefetch
 ```
 
 ## Manual update