restructured the add to app samples (#698)

add_to_app/prebuilt_module/README.md

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+# prebuild_module
+
+Embeds a full screen instance of Flutter as a prebuilt library that can be
+loaded into an existing iOS or Android app.
+
+## Description
+
+These apps are essentially identical to `android_fullscreen` and
+`ios_fullscreen`, respectively, with one key difference. Rather than being set
+up to compile the `flutter_module` from source each time the app is built, they
+import a the module as a prebuilt `aar` (Android) or framework (iOS). This can
+be useful for teams that don't want to require every developer working on the
+app to have the Flutter toolchain installed on their local machines.
+
+Prior to building either project for the first time, the `flutter_module` needs
+to be built.
+
+**Building for `android_using_prebuilt_module`**
+
+To build `flutter_module` as an aar, run this command from the `flutter_module`
+directory:
+
+```bash
+flutter build aar
+```
+
+It will produce `aar` files for debug, profile, and release mode. The Android
+app is configured to import the appropriate `aar` based on its own build
+configuration, so if you build a debug version of the app, it will look
+for the debug `aar`, and so on.
+
+If the `flutter_module` project is updated, the `aar` files must be rebuilt via
+one of the commands above in order for those changes to appear in the app.
+
+**Building for `ios_using_prebuilt_module`**
+
+To build `flutter_module` as a set of frameworks, run this command from the
+`flutter_module` directory:
+
+```bash
+flutter build ios-framework --xcframework --output=../ios_using_prebuilt_module/Flutter
+```
+
+This will output frameworks for debug, profile, and release modes into
+`ios_using_prebuilt_module/Flutter`. The project file for
+`ios_using_prebuilt_module` has been configured to find the frameworks there.
+
+For more information on how to modify an existing iOS app to reference prebuilt
+Flutter frameworks, see this article in the Flutter GitHub wiki:
+
+https://flutter.dev/docs/development/add-to-app/ios/project-setup
+
+## 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:
+  flutter build aar
+  open -a "Android Studio" ../android_using_prebuilt_module/ # macOS only
+  # Or open the ../android_using_prebuilt_module folder in Android Studio for
+  # other platforms.
+
+  # For iOS builds:
+  flutter build ios-framework --xcframework --output=../ios_using_prebuilt_module/Flutter
+  open ../ios_using_prebuilt_module/IOSUsingPrebuiltModule.xcodeproj
+```
+
+## Requirements
+
+* Flutter
+* Android
+  * Android Studio
+* iOS
+  * Xcode
+  * Cocoapods
+
+## Questions/issues
+
+See [add_to_app/README.md](../README.md) for further help.