Could you please add installation instruction for flutter?

[z2228391]

For example:
Use this package as a library

1. Depend on it
   Add this to your package's pubspec.yaml file:

dependencies:
flutter_webview_plugin: ^0.3.0+2

2. Install it
   You can install packages from the command line:

with Flutter:

$ flutter packages get

Alternatively, your editor might support flutter packages get. Check the docs for your editor to learn more.

3. Import it
   Now in your Dart code, you can use:

import 'package:flutter_webview_plugin/flutter_webview_plugin.dart';

[marcusfelix]

Added