Revert "Remove iOS instructions from the maps plugin readme. (flutter#744)"

This reverts commit 60252de.

Author: andreidiaconu

packages/google_maps_flutter/README.md

@@ -30,6 +30,8 @@ as a [dependency in your pubspec.yaml file](https://flutter.io/platform-plugins/
 
 Get an API key at <https://cloud.google.com/maps-platform/>.
 
+### Android
+
 Specify your API key in the application manifest `android/app/src/main/AndroidManifest.xml`:
 
 ```xml
@@ -39,6 +41,28 @@ Specify your API key in the application manifest `android/app/src/main/AndroidMa
                android:value="YOUR KEY HERE"/>
 ```
 
+### iOS
+
+Supply your API key in the application delegate `ios/Runner/AppDelegate.m`:
+
+```objectivec
+#include "AppDelegate.h"
+#include "GeneratedPluginRegistrant.h"
+#import "GoogleMaps/GoogleMaps.h"
+
+@implementation AppDelegate
+
+- (BOOL)application:(UIApplication *)application
+    didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
+  [GMSServices provideAPIKey:@"YOUR KEY HERE"];
+  [GeneratedPluginRegistrant registerWithRegistry:self];
+  return [super application:application didFinishLaunchingWithOptions:launchOptions];
+}
+@end
+```
+
+### Both
+
 You can now add a `GoogleMap` widget to your widget tree.
 
 The map view can be controlled with the `GoogleMapController` that is passed to