Added "s&box" to the Game Engine collection.

Added Facepunch's open source Source 2 engine "s&box" due to it's recent popularity spike.

Author: gunnar-hensel

collections/game-engines/index.md

@@ -59,7 +59,8 @@ items:
  - defold/defold
  - openfl/openfl
  - stride3d/stride
- - B4uti4github/choppy-js 
+ - B4uti4github/choppy-js
+ - Facepunch/sbox-public
 display_name: Game Engines
 created_by: leereilly
 ---