add example of scheduling callback in class method

[Amila Welihinda]

// --snip--
method poll(mut cx) {
  let cb = cx.argument::<JsFunction>(0)?;
  let this = cx.this();
  let events = cx.borrow(&this, |emitter| Arc::clone(&emitter.events));
  let emitter = EventEmitterTask(events);

  emitter.schedule(cb);

  Ok(JsUndefined::new().upcast())
}
// --snip--

Taken from neon-bindings/neon#386