Document how to check msiexec exit codes.

ScheduleReboot/README.txt

@@ -1 +1,13 @@
-Dummy application for testing of reboot handling.
+Dummy installer for testing of reboot handling.
+The installer always returns ERROR_SUCCESS_REBOOT_REQUIRED (3010) to indicate that a reboot is required to complete the install.
+
+Doc: https://learn.microsoft.com/en-us/windows/win32/msi/error-codes
+
+
+### How check msiexec codes
+
+Run the following commands from a batch script:
+```
+start /wait msiexec.exe /i ScheduleReboot
+echo msiexec exit code: %ERRORLEVEL%
+```