Update index.md

Added OpenXRay game engine.

Author: ademe-del

collections/game-engines/index.md

@@ -31,6 +31,7 @@ items:
  - renpy/renpy
  - OpenRA/OpenRA
  - OpenRCT2/OpenRCT2
+ - OpenXRay/xray16
  - xenko3d/xenko
  - lance-gg/lance
  - panda3d/panda3d