Update README with current usage (flags)

[carstenhag]

Hey, the usage printed out on the 1.0.0 jar has more info than the README:

Usage:
java -jar coffee-gb.jar [OPTIONS] ROM_FILE

Available options:
  -d  --force-dmg                Emulate classic GB (DMG) for universal ROMs
  -c  --force-cgb                Emulate color GB (CGB) for all ROMs
  -b  --use-bootstrap            Start with the GB bootstrap
  -db --disable-battery-saves    Disable battery saves
      --debug                    Enable debug console
      --headless                 Start in the headless mode

[trekawek]

Good point, thanks!