Add an iOS app clip sample (#538)

ios_app_clip/README.md

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+# ios_app_clip
+
+A sample project demonstrating integration with iOS 14's App Clip.
+
+The App Clip target is rendered by Flutter and uses a plugin.
+
+## Intent
+
+1. Demonstrate it's possible to produce iOS App Clip targets rendered in Flutter.
+2. Serve as a canonical example to test the App Clip scenario.
+3. Let users validate their integration steps when following the guide at
+   [flutter.dev/docs/development/platform-integration/ios-app-clip](https://flutter.dev/docs/development/platform-integration/ios-app-clip)
+
+## To run
+
+In order to run.
+
+1. `cd ios`
+2. `pod install`
+3. `open Runner.xcworkspace`
+4. Inside Xcode, select the `AppClip` target from the scheme dropdown
+5. Select an iOS 14 device
+6. Select a development team as needed
+7. Press run in Xcode to run
+
+## Questions/issues
+
+If you have a general question about iOS App Clips in Flutter, the
+best places to go are:
+
+* [The FlutterDev Google Group](https://groups.google.com/forum/#!forum/flutter-dev)
+* [The Flutter Gitter channel](https://gitter.im/flutter/flutter)
+* [StackOverflow](https://stackoverflow.com/questions/tagged/flutter)
+
+If you run into an issue with the sample itself, please file an issue
+in the [main Flutter repo](https://github.com/flutter/flutter/issues).