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

add_to_app/books/ios_books/IosBooks/AppDelegate.swift

@@ -0,0 +1,37 @@
+// 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 Flutter
+import UIKit
+
+@main
+class AppDelegate: UIResponder, UIApplicationDelegate {
+  lazy var engine: FlutterEngine = {
+    let result = FlutterEngine.init(name: "Books")
+    // This could be `run` earlier in the app to avoid the overhead of doing it the first time the
+    // engine is needed.
+    result.run()
+    return result
+  }()
+
+  func application(
+    _ application: UIApplication,
+    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
+  ) -> Bool {
+    return true
+  }
+
+  func application(
+    _ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession,
+    options: UIScene.ConnectionOptions
+  ) -> UISceneConfiguration {
+    return UISceneConfiguration(
+      name: "Default Configuration", sessionRole: connectingSceneSession.role)
+  }
+
+  func application(
+    _ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>
+  ) {
+  }
+}