Add a Material/Cupertino adaptive application example (#69)

platform_design/README.md

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+# Platform Design
+
+Instead of transliterating widgets one by one between Cupertino and Material,
+Android and iOS apps often follow different information architecture patterns
+that require some design decisions.
+
+This sample project shows a Flutter app that maximizes application code reuse
+while adhering to different design patterns on Android and iOS. On
+Android, it uses Material's [lateral navigation](https://material.io/design/navigation/understanding-navigation.html#types-of-navigation)
+based on a drawer and on iOS, it adheres to Apple Human Interface Guideline's
+[flat navigation](https://developer.apple.com/design/human-interface-guidelines/ios/app-architecture/navigation/)
+by using a bottom tab bar.
+
+Visually, the app presents platform-agnostic content surrounded by
+platform-specific 'chrome'.
+
+# Preview
+
+![App's platform toggling preview](adaptive-overview.gif)
+
+See https://youtu.be/svhbbFZg1IA for a longer non-gif format.
+
+# Features
+
+## Home
+
+Defines the top level navigation structure of the app and shows the contents
+of the songs tab on launch.
+
+### Android
+
+* Uses the drawer paradigm on the root page.
+
+### iOS
+
+* Uses bottom tab bars with parallel navigation stacks.
+
+## Songs feed tab
+
+Shows platform-agnostic cards that is tappable and that performs a hero
+transition on top of the platform native page transitions.
+
+Both platforms also show a button in their app/nav bar to toggle the platform.
+
+### Android
+
+* Android uses a static pull-to-refresh pattern with an additional refresh
+button in the app bar.
+* The song details page must be popped in order to change tabs on Android.
+
+### iOS
+
+* The iOS songs tab uses a scrollable iOS 11 large title style navigation bar.
+* iOS uses an overscrolling pull-to-refresh pattern.
+* On iOS, parallel tabs are always accessible and the songs tab's navigation
+stack is preserved when changing tabs.
+
+## News Tab
+
+Shows platform-agnostic news boxes.
+
+### Android
+
+* The news tab always appears on top of the songs tab when summoned from the
+drawer.
+
+### iOS
+
+* The news tab appears instead of the songs tab on iOS when switching tabs from
+the tab bar.
+
+## Profile Tab
+
+Shows a number of user preferences.
+
+### Android
+
+* The profile tab appears on top of the songs tab on Android.
+* Has tappable preference cards which shows a multiple-choice dialog on Android.
+* The log out button shows a 2 button dialog on Android.
+
+### iOS
+
+* The profile tab appears instead of the songs tab on iOS.
+* Has tappable preference cards which shows a picker on iOS.
+* The log out button shows a 3 choice action sheet on iOS.
+
+## Settings Tab
+
+Shows a number of app settings via Material switches which auto adapt to the
+platform.
+
+### Android
+
+* The settings is directly available in the drawer on Android since a Material
+Design drawer can fit many tabs.
+
+### iOS
+
+* The settings is accessible from a button inside the profile tab's nav bar on
+iOS. This is a common pattern since there are conventionally more items in the
+drawer than there are tabs.
+* On iOS, the settings page is shown as a full screen dialog instead of a tab
+in the tab scaffold.