Adds add2app Flutter module and an iOS project to run it. (#126)

experimental/add_to_app/README.md

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+# Add-to-App Sample
+
+***The Add-to-App sample is designed to build with Flutter's `master` channel. See
+the [README](../README.md) in the `experimental` directory for details.***
+
+This directory contains a bunch of Android and iOS projects that each import
+a standalone Flutter module called `example_module`.
+
+## Goals for this sample
+
+* Show developers how to add Flutter to their existing applications.
+
+## The important bits
+
+### `example_module`
+
+This is the Flutter module that'll be added to all the individual projects
+in the `add2app` folder.
+
+### `SimpleIOSExample`
+
+An iOS application that imports `example_module` and uses it to display
+a `FlutterViewController` containing widgets from the module. This project
+uses CocoaPods, so prior to running it for the first time, use this command
+to set up the workspace file:
+
+```bash
+cd SimpleIOSExample
+pod install
+```
+
+## Questions/issues
+
+If you have a general question about incorporating Flutter into an existing
+iOS or Android app, 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).
+