added sample code for scaffold widget [flutter#21136]

packages/flutter/lib/src/material/scaffold.dart

@@ -662,6 +662,41 @@ class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTr
 /// [ScaffoldState] for the current [BuildContext] via [Scaffold.of] and use the
 /// [ScaffoldState.showSnackBar] and [ScaffoldState.showBottomSheet] functions.
 ///
+/// ## Sample code
+///
+/// This example shows a [Scaffold] with an [AppBar], a [BottomAppBar]
+/// and a [FloatingActionButton]. The [body] is a [Text] placed in a [Center]
+/// in order to center the text within the [Scaffold] and the
+/// [FloatingActionButton] is centered and docked within the
+/// [BottomAppBar] using [FloatingActionButtonLocation.centerDocked].
+///
+/// ```dart
+/// Scaffold(
+///      appBar: AppBar(
+///        title: Text('Sample Code'),
+///      ),
+///      body: Center(
+///        child: Text(
+///          'Scaffold',
+///        ),
+///      ),
+///
+///      bottomNavigationBar: BottomAppBar(
+///        child: Container(
+///          height: 50.0,
+///        ),
+///      ),
+///
+///      floatingActionButton: FloatingActionButton(
+///        onPressed: () {},
+///        tooltip: 'Increment',
+///        child: Icon(Icons.add),
+///      ),
+///
+///      floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
+///    )
+/// ```
+///
 /// See also:
 ///
 ///  * [AppBar], which is a horizontal bar typically shown at the top of an app