restructured the add to app samples (#698)

add_to_app/fullscreen/README.md

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+# fullscreen
+
+Embeds a full screen instance of Flutter into an existing iOS or Android app.
+
+## Description
+
+These apps showcase a relatively straightforward integration of
+`flutter_module`:
+
+* The Flutter module is built along with the app when the app is built.
+* The Flutter engine is warmed up at app launch.
+* The Flutter view is presented with a full-screen Activity or
+  UIViewController.
+* The Flutter view is a navigational leaf node; it does not launch any new,
+  native Activities or UIViewControllers in response to user actions.
+
+If you are new to Flutter's add-to-app APIs, these projects are a great place
+to begin learning how to use them.
+
+## 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/
+  flutter pub get
+
+  # For Android builds:
+  open -a "Android Studio" ../android_fullscreen # macOS only
+  # Or open the ../android_fullscreen folder in Android Studio for other platforms.
+
+  # For iOS builds:
+  cd ../ios_fullscreen
+  pod install
+  open IOSFullScreen.xcworkspace
+```
+
+## Requirements
+
+* Flutter
+* Android
+  * Android Studio
+* iOS
+  * Xcode
+  * Cocoapods
+
+## Questions/issues
+
+See [add_to_app/README.md](../README.md) for further help.