docs: document the wrapper/example app structure

documentation/CONTRIBUTING.md

@@ -2,6 +2,23 @@
 
 I welcome and encourage all pull requests. It usually takes 24-48 hours to respond to any issue or request.
 
+## Example app structure
+
+Every example app uses the same structure, so the code that teaches a concept is
+easy to find:
+
+- `lib/main.dart`: the app wrapper. It holds only `runApp()` and a `MyApp` widget
+  that returns the `MaterialApp` (title, theme, routes). Keep this file minimal and
+  uniform across apps.
+- `lib/example.dart`: the focused example code. It contains the full screen
+  (`Scaffold` + `AppBar`) and the widget that demonstrates the concept, with a
+  `// Example: <what this app teaches>` comment at the top.
+- Supporting files (screens, tabs, services, models, widgets, utils) sit alongside
+  the example and are imported by `lib/example.dart`.
+
+To understand an example app, read `lib/example.dart`. That file holds the concept;
+`lib/main.dart` holds only the wrapping boilerplate.
+
 ## Basic rules
 
 1. Match coding style (braces, spacing, etc.). Use the `Reformat Code` feature of Android Studio (`CMD` + `Option` + `L` on Mac, `CTRL` + `ALT` + `L` on Linux and Windows).
@@ -11,3 +28,7 @@ I welcome and encourage all pull requests. It usually takes 24-48 hours to respo
 1. Check for existing [issues](https://github.com/nisrulz/flutter-examples/issues) before filing a new one.
 1. Follow the same standard as all other projects in this repo.
    > The package name must conform to the format: `github.nisrulz.<app_name>`
+
+When you add a new example app, keep the wrapper in `lib/main.dart` and the focused
+example code in `lib/example.dart`. Run `make docs`, `make analyze`, and `make doctor`
+before opening a pull request.