[in_app_purchase] Add documentation for price change confirmations

packages/in_app_purchase/in_app_purchase/README.md

@@ -247,6 +247,55 @@ InAppPurchase.instance
     .buyNonConsumable(purchaseParam: purchaseParam);
 ```
 
+### Confirming subscription price changes
+
+When you change the price of a subscription the user will need to confirm the price change. If the user does not 
+confirm the price change the subscription will not be auto-renewed. On both iOS and Android the user will 
+automatically get a popup to confirm the price change, but you as a developer can show the popup on a moment that 
+suits you better. This works different on the Apple App Store and on the Google Play Store.
+
+#### Google Play Store (Android)
+When you raise the subscription price you will have 7 days to ask the user to approve the new price. The official 
+documentation can be found [here](https://support.google.com/googleplay/android-developer/answer/140504?hl=en#zippy=%2Cprice-changes).
+When you lower the price the user will automatically receive the lower price and does not have to approve the price 
+change.
+
+After 7 days the user will be notified through email and notifications on Google Play to agree with the new price, 
+so you have 7 days to explain the user in your app that the price is going to change and ask them to accept the 
+change. You will have to keep track of whether or not the price change is already accepted in your app or in the 
+backend. The [Google Play API](https://developers.google.com/android-publisher/api-ref/rest/v3/purchases.subscriptions) 
+can be used to check whether or not the price change is accepted by the user by reading the priceChange property on 
+a subscription object.
+
+The InAppPurchaseAndroidPlatformAddition can be used to show the price change confirmation flow. The additions 
+contain the function `launchPriceChangeConfirmationFlow` which needs the sku code of the subscription. 
+
+```dart
+//import for InAppPurchaseAndroidPlatformAddition
+import 'package:in_app_purchase_android/in_app_purchase_android.dart';
+//import for BillingResponse
+import 'package:in_app_purchase_android/billing_client_wrappers.dart';
+
+if (Platform.isAndroid) {
+  final InAppPurchaseAndroidPlatformAddition androidAddition =
+    _inAppPurchase
+      .getPlatformAddition<InAppPurchaseAndroidPlatformAddition>();
+  var priceChangeConfirmationResult = 
+      await androidAddition.launchPriceChangeConfirmationFlow(
+    sku: 'purchaseId',
+  );
+  if (priceChangeConfirmationResult.responseCode == BillingResponse.ok){
+    // TODO acknowledge price change
+  }else{
+    // TODO show error
+  }
+}
+```
+
+#### Apple App Store (iOS)
+
+//TODO
+
 ### Accessing platform specific product or purchase properties
 
 The function `_inAppPurchase.queryProductDetails(productIds);` provides a `ProductDetailsResponse` with a 

packages/in_app_purchase/in_app_purchase/example/lib/main.dart

@@ -438,6 +438,26 @@ class _MyAppState extends State<_MyApp> {
     });
   }
 
+  Future<void> confirmPriceChange() async {
+    if (Platform.isAndroid) {
+      final InAppPurchaseAndroidPlatformAddition androidAddition =
+          _inAppPurchase
+              .getPlatformAddition<InAppPurchaseAndroidPlatformAddition>();
+      var priceChangeConfirmationResult =
+          await androidAddition.launchPriceChangeConfirmationFlow(
+        sku: 'purchaseId',
+      );
+      if (priceChangeConfirmationResult.responseCode == BillingResponse.ok) {
+        // TODO acknowledge price change
+      } else {
+        // TODO show error
+      }
+    }
+    if(Platform.isIOS){
+      //TODO setup flow
+    }
+  }
+
   GooglePlayPurchaseDetails? _getOldSubscription(
       ProductDetails productDetails, Map<String, PurchaseDetails> purchases) {
     // This is just to demonstrate a subscription upgrade or downgrade.

packages/in_app_purchase/in_app_purchase/pubspec.yaml

@@ -20,7 +20,7 @@ dependencies:
   flutter:
     sdk: flutter
   in_app_purchase_platform_interface: ^1.0.0
-  in_app_purchase_android: ^0.1.0
+  in_app_purchase_android: ^0.1.4
   in_app_purchase_ios: ^0.1.0
 
 dev_dependencies: