Add Swift example to README.md for google_maps_flutter (#1161)

Author: aoinakanishi

packages/google_maps_flutter/README.md

@@ -57,6 +57,25 @@ Specify your API key in the application delegate `ios/Runner/AppDelegate.m`:
 @end
 ```
 
+Or in your swift code, specify your API key in the application delegate `ios/Runner/AppDelegate.swift`:
+
+```swift
+import UIKit
+import Flutter
+import GoogleMaps
+
+@UIApplicationMain
+@objc class AppDelegate: FlutterAppDelegate {
+  override func application(
+    _ application: UIApplication,
+    didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?
+  ) -> Bool {
+    GMSServices.provideAPIKey("YOUR KEY HERE")
+    GeneratedPluginRegistrant.register(with: self)
+    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
+  }
+}
+```
 Opt-in to the embedded views preview by adding a boolean property to the app's `Info.plist` file
 with the key `io.flutter.embedded_views_preview` and the value `YES`.