Add in_app_purchase to /readme (firebase#1634)

README.md

@@ -47,6 +47,7 @@ These are the available plugins in this repository.
 | [google_maps_flutter](./packages/google_maps_flutter) | [![pub package](https://img.shields.io/pub/v/google_maps_flutter.svg)](https://pub.dev/packages/google_maps_flutter) |
 | [google_sign_in](./packages/google_sign_in/) | [![pub package](https://img.shields.io/pub/v/google_sign_in.svg)](https://pub.dev/packages/google_sign_in) |
 | [image_picker](./packages/image_picker/) | [![pub package](https://img.shields.io/pub/v/image_picker.svg)](https://pub.dev/packages/image_picker) |
+| [in_app_purchase](./packages/in_app_purchase/) | [![pub package](https://img.shields.io/pub/v/in_app_purchase.svg)](https://pub.dev/packages/in_app_purchase) |
 | [local_auth](./packages/local_auth/) | [![pub package](https://img.shields.io/pub/v/local_auth.svg)](https://pub.dev/packages/local_auth) |
 | [package_info](./packages/package_info/) | [![pub package](https://img.shields.io/pub/v/package_info.svg)](https://pub.dev/packages/package_info) |
 | [path_provider](./packages/path_provider/) | [![pub package](https://img.shields.io/pub/v/path_provider.svg)](https://pub.dev/packages/path_provider) |