Add comment regarding the use of RoughnessMipmapper

examples/webgl_loader_gltf.html

@@ -57,6 +57,7 @@
 
 						// model
 
+						// use of RoughnessMipmapper is optional
 						var roughnessMipmapper = new RoughnessMipmapper( renderer );
 
 						var loader = new GLTFLoader().setPath( 'models/gltf/DamagedHelmet/glTF/' );