added gpu particle system to examples listing, added additional comments

examples/index.html

@@ -236,6 +236,7 @@
 				"webgl_geometry_text",
 				"webgl_geometry_text2",
 				"webgl_gpgpu_birds",
+				"webgl_gpu_particle_system",
 				"webgl_hdr",
 				"webgl_helpers",
 				"webgl_interactive_buffergeometry",

examples/webgl_gpu_particle_system.html

@@ -52,7 +52,10 @@
 
       scene = new THREE.Scene();
 
-      // setup particle system
+      // The GPU Particle system extends THREE.Object3D, and so you can use it
+      // as you would any other scene graph component.  Particle positions will be
+      // relative to the position of the particle system, but you will probably only need one
+      // system for your whole scene
       particleSystem = new THREE.GPUParticleSystem({
         maxParticles: 250000
       });