Added sample code for multiple flutters on iOS (#670)

add_to_app/multiple_flutters/README.md

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+# multiple_flutters
+
+This is a sample that shows how to use the Flutter Engine Groups API to embed
+multiple instances of Flutter into an existing Android or iOS project.
+
+## Getting Started
+
+For iOS instructions, see:
+[multiple_flutters_ios](./multiple_flutters_ios/README.md).
+
+For Android instructions, see:
+[multiple_flutters_android](./multiple_flutters_android/README.md).
+
+## Requirements
+
+* Flutter -- after version 1991216543eb74e07c2de3746ca07c92071b19ac
+* Android
+  * Android Studio
+* iOS
+  * Xcode
+  * Cocoapods
+
+## Flutter Engine Group
+
+These examples use the Flutter Engine Group APIs on the host platform which
+allows engines to share memory and CPU intensive resources. This leads to easier
+embedding of Flutter into an existing app since multiple entrypoints can be
+maintained via a FlutterFragment on Android or a UIViewController on iOS.
+Before FlutterEngineGroup, users had to juggle the usage of a small number of
+engines judiciously.
+
+More info on those API's can be found in the source
+code:
+
+- iOS -
+  [FlutterEngineGroup.h](https://github.com/flutter/engine/blob/master/shell/platform/darwin/ios/framework/Headers/FlutterEngineGroup.h)
+- Android -
+  [FlutterEngineGroup.java](https://github.com/flutter/engine/blob/master/shell/platform/android/io/flutter/embedding/engine/FlutterEngineGroup.java)
+
+## Important Files
+
+### iOS
+
+- [DataModel.swift](./multiple_flutters_ios/MultipleFluttersIos/HostViewController.swift)
+  — Observable data model that is the source of truth for the host platform and Flutter.
+- [HostViewController.swift](./multiple_flutters_ios/MultipleFluttersIos/HostViewController.swift)
+  — A UIViewController that synchronizes to the DataModel.
+- [main.dart](./multiple_flutters_module/lib/main.dart) — The Flutter source
+  code.
+- [SingleFlutterViewController.swift](./multiple_flutters_ios/MultipleFluttersIos/SingleFlutterViewController.swift)
+  — A subclass of FlutterViewController that synchronizes with the DataModel.
+- [DoubleFlutterViewController.swift](./multiple_flutters_ios/MultipleFluttersIos/DoubleFlutterViewController.swift)
+  — A UIViewController that embeds multiple Flutter instances.
+
+### Android
+
+## Data Synchronization Description
+
+This sample code performs data synchronization to share data between the host
+platform and multiple instances of Flutter by combining the
+[Observer design pattern](https://en.wikipedia.org/wiki/Observer_pattern) and
+[Flutter platform channels](https://flutter.dev/docs/development/platform-integration/platform-channels).
+Here is how it works:
+
+- The definitive source of truth for the data lives in the host platform data
+  model.
+- Every host view displaying Flutter content maintains: a Flutter engine, a
+  bidirectional platform channel, and a subscription to the host data model.
+- Flutter instances maintain a copy of the data they are interested in reading,
+  and this data is seeded by the host when the instance is first displayed.
+- Mutations from Flutter code are sent to the host platform via the channel. The
+  host platform performs the mutations, and then notifies all host view
+  controllers and Flutter engines of the new value.
+- Mutations from host code happen directly on the data model who notifies host
+  view controllers and Flutter engines of the new value.
+
+This is just one possible way to synchronize the data between the host platform
+and multiple Flutter instances. A more complete implementation is proposed in
+the work of
+[flutter/issues/72030](https://github.com/flutter/flutter/issues/72030).