Add README instructions for enablePendingPurchases (flutter#2921)

Add README instructions for enablePendingPurchases.

If a developer attempts to access this plugin on Android without invoking enablePendingPurchases, an exception is thrown.

Related StackOverflow post:
https://stackoverflow.com/questions/60046903/flutter-in-app-purchase-enablependingpurchases-enablependingpurchases-must

Author: matthew-carroll

Diff for: packages/in_app_purchase/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.3.4+5
+
+* Added necessary README docs for getting started with Android.
+
 ## 0.3.4+4
 
 * Update package:e2e -> package:integration_test

Diff for: packages/in_app_purchase/README.md

@@ -51,6 +51,19 @@ use.
 
 ### Initializing the plugin
 
+```dart
+void main() {
+  // Inform the plugin that this app supports pending purchases on Android.
+  // An error will occur on Android if you access the plugin `instance`
+  // without this call.
+  //
+  // On iOS this is a no-op.
+  InAppPurchaseConnection.enablePendingPurchases();
+  
+  runApp(MyApp());
+}
+```
+
 ```dart
 // Subscribe to any incoming purchases at app initialization. These can
 // propagate from either storefront so it's important to listen as soon as
@@ -90,7 +103,7 @@ if (!available) {
 // Set literals require Dart 2.2. Alternatively, use `Set<String> _kIds = <String>['product1', 'product2'].toSet()`.
 const Set<String> _kIds = {'product1', 'product2'};
 final ProductDetailsResponse response = await InAppPurchaseConnection.instance.queryProductDetails(_kIds);
-if (!response.notFoundIDs.isEmpty) {
+if (response.notFoundIDs.isNotEmpty) {
     // Handle the error.
 }
 List<ProductDetails> products = response.productDetails;

Diff for: packages/in_app_purchase/pubspec.yaml

@@ -1,7 +1,7 @@
 name: in_app_purchase
 description: A Flutter plugin for in-app purchases. Exposes APIs for making in-app purchases through the App Store and Google Play.
 homepage: https://github.com/flutter/plugins/tree/master/packages/in_app_purchase
-version: 0.3.4+4
+version: 0.3.4+5
 
 dependencies:
   async: ^2.0.8