added usage example for BannerAd

[dyazincahya]

import { BannerAdSize } from "@nativescript/firebase-admob";

// example function for layoutChanged
export function bannerLoaded(args) {
  const banner = args.object;
  const adSize = new BannerAdSize(100, 100);
  banner.size = adSize;
  banner.load();
}

[rigor789]

Thanks for the change, this file is auto-generated from the plugins repo README, so I went ahead and applied your change to the plugin's own README here: NativeScript/firebase@7712c1b

[dyazincahya]

Okay