Update WebGLRenderer.js

Add forum link to deprecation warning.

src/renderers/WebGLRenderer.js

@@ -2446,14 +2446,14 @@ class WebGLRenderer {
 
 	get useLegacyLights() { // @deprecated, r155
 
-		console.warn( 'THREE.WebGLRenderer: The property .useLegacyLights has been deprecated. Migrate your lighting according to the following guide: TODO.' );
+		console.warn( 'THREE.WebGLRenderer: The property .useLegacyLights has been deprecated. Migrate your lighting according to the following guide: https://discourse.threejs.org/t/updates-to-lighting-in-three-js-r155/53733.' );
 		return this._useLegacyLights;
 
 	}
 
 	set useLegacyLights( value ) { // @deprecated, r155
 
-		console.warn( 'THREE.WebGLRenderer: The property .useLegacyLights has been deprecated. Migrate your lighting according to the following guide: TODO.' );
+		console.warn( 'THREE.WebGLRenderer: The property .useLegacyLights has been deprecated. Migrate your lighting according to the following guide: https://discourse.threejs.org/t/updates-to-lighting-in-three-js-r155/53733.' );
 		this._useLegacyLights = value;
 
 	}