Added iOS example for add-to-app Pigeon (#722)

add_to_app/books/ios_books/IosBooks/SceneDelegate.swift

@@ -0,0 +1,32 @@
+// Copyright 2020 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import UIKit
+
+class SceneDelegate: UIResponder, UIWindowSceneDelegate {
+
+  var window: UIWindow?
+
+  func scene(
+    _ scene: UIScene, willConnectTo session: UISceneSession,
+    options connectionOptions: UIScene.ConnectionOptions
+  ) {
+    guard let _ = (scene as? UIWindowScene) else { return }
+  }
+
+  func sceneDidDisconnect(_ scene: UIScene) {
+  }
+
+  func sceneDidBecomeActive(_ scene: UIScene) {
+  }
+
+  func sceneWillResignActive(_ scene: UIScene) {
+  }
+
+  func sceneWillEnterForeground(_ scene: UIScene) {
+  }
+
+  func sceneDidEnterBackground(_ scene: UIScene) {
+  }
+}