Update migration section of README.md

Signed-off-by: George Pisaltu <[email protected]>

Author: georgepisaltu

README.md

@@ -39,6 +39,8 @@ code returned by `casper-node`:
   * If 0 (success), runs the same version of `casper-node` in validator mode
   * If 102 (downgrade), searches for the immediate previous installed version of `casper-node` and runs it in validator
     mode
+  * If 103 (shutdown), runs the script at `/etc/casper/casper_shutdown_script` if present and exits with its exit code,
+    otherwise exits with `0`.
   * Any other value causes the launcher to exit with an error
 
 If the launcher cannot find an appropriate version at any stage of upgrading or downgrading, it exits with an error.