add three.js example to README

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: KilledByAPixel

README.md

@@ -131,6 +131,7 @@ LittleJS comes with several demos both for learning and using as starter project
 - [Puzzle Game](https://killedbyapixel.github.io/LittleJS/examples/puzzle/) - Match 3 puzzle game with HD rendering and high score tracking
 - [Platformer](https://killedbyapixel.github.io/LittleJS/examples/platformer/) - Platformer/shooter demo that loads level data
 - [Box2D Demo](https://killedbyapixel.github.io/LittleJS/examples/box2d/) - Box2D plugin demonstration and testbed
+- [Three.js Demo](https://killedbyapixel.github.io/LittleJS/examples/threejs/) - 3D rendering with Three.js behind the 2D canvas
 - [Stress Test](https://killedbyapixel.github.io/LittleJS/examples/stress/) - Sprite rendering benchmark and music system demo
 - [Particle System Designer](https://killedbyapixel.github.io/LittleJS/examples/particles/) - Particle system editor and visualizer