restructured the add to app samples (#698)

add_to_app/plugin/README.md

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+# plugin
+
+Embeds a full screen Flutter instance that is using plugins into an existing iOS
+or Android app.
+
+## Description
+
+These apps are similar to the samples in [`fullscreen`](../fullscreen), with the
+following differences:
+
+* They include the native code (Kotlin or Swift) required to initialize plugins
+  at Flutter engine creation time.
+* Their Flutter view includes an additional button that opens the Flutter docs
+  in the mobile device's browser.
+
+If you're interested in learning what additional steps an app needs to take in
+order to use a Flutter module that relies on plugins, these projects can help.
+
+## tl;dr
+
+If you're just looking to get up and running quickly, these bash commands will
+fetch packages and set up dependencies (note that the above commands assume
+you're building for both iOS and Android, with both toolchains installed):
+
+```bash
+  #!/bin/bash
+  set -e
+
+  cd flutter_module_using_plugin
+  flutter pub get
+
+  # For Android builds:
+  open -a "Android Studio" ../android_using_plugin # macOS only
+  # Or open the ../android_using_plugin folder in Android Studio for other platforms.
+
+  # For iOS builds:
+  cd ../ios_using_plugin
+  pod install
+  open IOSUsingPlugin.xcworkspace
+```
+
+## Requirements
+
+* Flutter
+* Android
+  * Android Studio
+* iOS
+  * Xcode
+  * Cocoapods
+
+## Questions/issues
+
+See [add_to_app/README.md](../README.md) for further help.