restructured the add to app samples (#698)

add_to_app/books/flutter_module_books/README.md

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+# Books add-to-app sample
+
+This application simulates a mock scenario in which an existing app with
+business logic and middleware already exists. It demonstrates how to add Flutter
+to an app that has established patterns for these domains. For more information
+on how to use it, see the [README.md](../README.md) parent directory.
+
+This application also utilizes the [Pigeon](https://pub.dev/packages/pigeon)
+plugin to avoid manual platform channel wiring. Pigeon autogenerates the
+platform channel code in Dart/Java/Objective-C to allow interop using higher
+order functions and data classes instead of string-encoded methods and
+serialized primitives.
+
+The Pigeon autogenerated code is checked-in and ready to use. If the schema
+in `pigeon/schema.dart` is updated, the generated classes can also be re-
+generated using:
+
+```shell
+flutter pub run pigeon \
+    --input pigeon/schema.dart \
+    --java_out ../android_books/app/src/main/java/dev/flutter/example/books/Api.java \
+    --java_package "dev.flutter.example.books"
+```
+
+## Demonstrated concepts
+
+* An existing books catalog app is already implemented in Kotlin and Swift.
+* The platform-side app has existing middleware constraints that should also
+  be the middleware foundation for the additional Flutter screen.
+    * On Android, the Kotlin app already uses GSON and OkHttp for networking and
+      references the Google Books API as a data source. These same libraries
+      also underpin the data fetched and shown in the Flutter screen.
+* The platform application interfaces with the Flutter book details page using
+  idiomatic platform API conventions rather than Flutter conventions.
+    * On Android, the Flutter activity receives the book to show via activity
+      intent and returns the edited book by setting the result intent on the
+      activity. No Flutter concepts are leaked into the consumer activity.
+* The [pigeon](https://pub.dev/packages/pigeon) plugin is used to generate
+  interop APIs and data classes. The same `Book` model class is used within the
+  Kotlin/Swift program, the Dart program and in the interop between Kotlin/Swift
+  and Dart.
+
+## More info
+
+For more information about Flutter, check out
+[flutter.dev](https://flutter.dev).
+
+For instructions on how to integrate Flutter modules into your existing
+applications, see Flutter's
+[add-to-app documentation](https://flutter.dev/docs/development/add-to-app).