chore: add flutter quickstart example, assets and update README

examples/flutter/quickstart/ios/Runner/AppDelegate.swift

@@ -0,0 +1,13 @@
+import Flutter
+import UIKit
+
+@main
+@objc class AppDelegate: FlutterAppDelegate {
+  override func application(
+    _ application: UIApplication,
+    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
+  ) -> Bool {
+    GeneratedPluginRegistrant.register(with: self)
+    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
+  }
+}