Add documentation for the -display command line argument in USAGE.md

Author: sebholt

USAGE.md

@@ -87,6 +87,7 @@ Here brief usage of command line options, supported by game.
 | Option               | Type    | Default                             | Platform | Description                                                       |
 |----------------------|---------|-------------------------------------|----------|-------------------------------------------------------------------|
 | `-dedicated`, `-d`   | boolean | Off                                 | all      | Run game in dedicated mode                                        |
+| `-display`           | integer | 0                                   | all      | Run game on the selected display                                  |
 | `-fullscreen`, `-f`  | boolean | On                                  | all      | Run game in fullscreen mode                                       |
 | `-logfile`           | boolean | Off                                 | all      | Enable file logging to Descent3.log                               |
 | `-loglevel <LEVEL>`  | string  | INFO (on Release), DEBUG (on Debug) | all      | Set log level (NONE, VERBOSE, DEBUG, INFO, WARNING, ERROR, FATAL) |