Add parseAsync to GLTFExporter doc

Author: trusktr

docs/examples/en/exporters/GLTFExporter.html

@@ -148,6 +148,15 @@ <h3>[method:undefined parse]( [param:Object3D input], [param:Function onComplete
 		Generates a .gltf (JSON) or .glb (binary) output from the input (Scenes or Objects)
 		</p>
 
+		<h3>[method:Promise<ArrayBufferOrJSONObject> parseAsync]( [param:Object3D input], [param:Object options] )</h3>
+
+		<p>
+			Generates a .gltf (JSON) or .glb (binary) output from the input (Scenes or Objects). This is just like the
+			[page:parse parse]() method, but instead of accepting callbacks it returns a promise that resolves with the
+			result, and otherwise accepts the same options. Use try-catch around the promise, or <code>promise.catch()</code>,
+			to catch errors.
+		</p>
+
 		<h2>Source</h2>
 
 		<p>