add contributing doc

packages/webview_flutter/webview_flutter_android/CONTRIBUTING.md

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+# Contributing to `webview_flutter_android`
+
+Please start by taking a look at the general guide to contributing to the `flutter/packages` repo:
+https://github.com/flutter/packages/blob/main/CONTRIBUTING.md
+
+## Package Structure
+
+This plugin serves as a platform implementation plugin as outlined in [federated plugin](https://docs.flutter.dev/packages-and-plugins/developing-packages#federated-plugins). 
+The sections below will provide an overview of how this plugin implements this portion with Android.
+
+For making changes to this package, please take a look at [changing federated plugin](https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#changing-federated-plugins).
+
+### Quick Overview
+
+This plugin implements the platform interface provided in `webview_flutter_platform_interface` using
+the native WebKit APIs for Android.
+
+#### SDK Wrappers
+
+To access native APIS, this plugins uses Dart wrappers of the native library. The native library is
+wrapped using using the `ProxyApi` feature from the `pigeon` package.
+
+The wrappers for the native library can be updated and modified by changing `pigeons/android_webview.dart`.
+
+The generated files are located:
+* `lib/src/android_webkit.g.dart`
+* `android/src/main/java/io/flutter/plugins/webviewflutter/AndroidWebkitLibrary.g.kt`
+
+To update a wrapper for a platform, follow the steps:
+
+##### 1. Ensure the project has been built at least once
+
+Run `flutter build apk --debug` in `example/`.
+
+##### 2. Make changes to the respective pigeon file that matches the native SDK
+
+* Android Dependency: https://developer.android.com/reference/android/webkit/package-summary
+* Pigeon file to update: `pigeons/android_webview.dart`
+
+##### 3. Run the code generator from the terminal
+
+Run: `dart run pigeon --input pigeons/interactive_media_ads_android.dart`
+
+##### 4. Update the generated APIs in native code
+
+Running the `flutter build` command from step 1 again should provide build errors and indicate what
+needs to be done. Alternatively, it can be easier to update native code with the platform's specific
+IDE:
+
+Open `example/android/` in a separate Android Studio project.
+
+##### 5. Write API tests
+
+Assuming a non-static method or constructor was added to the native wrapper, a native test will need
+to be added.
+
+Tests location: `android/src/test/java/io/flutter/plugins/webviewflutter/`
+
+#### Dart Unit Testing
+
+Tests for the platform implementations use [mockito] to generate mock objects of the native Dart
+wrappers. To generate the mock objects in `test/`, run
+`dart run build_runner build --delete-conflicting-outputs`.
+
+#### 
+
+## Recommended Process for Adding a New Feature
+
+### 1. Create a new feature request issue in the `flutter/flutter` repo.
+
+See https://github.com/flutter/flutter/issues/new?assignees=&labels=&projects=&template=3_feature_request.yml
+
+### 2. In that issue add the specific native classes/methods that this feature requires for each platform:
+
+Add a note if this feature only exist for a single platform.
+
+### 3. Add a design where the feature can be added to the platform interface and app-facing interface.
+
+If this is only supported on a single platform, add where it can be added in the platform
+implementation.
+
+### 4. Work can be started on the feature request or you can wait for feedback from a Flutter contributor.
+
+[IMA SDKs]: https://developers.google.com/interactive-media-ads