improve the default help to include common actions (#5980)

packages/flutter_tools/lib/src/runner/flutter_command_runner.dart

@@ -28,7 +28,16 @@ const String kFlutterEnginePackageName = 'sky_engine';
 class FlutterCommandRunner extends CommandRunner {
   FlutterCommandRunner({ bool verboseHelp: false }) : super(
     'flutter',
-    'Manage your Flutter app development.'
+    'Manage your Flutter app development.\n'
+      '\n'
+      'Common actions:\n'
+      '\n'
+      '  flutter create <output directory>\n'
+      '    Create a new Flutter project in the specified directory.\n'
+      '\n'
+      '  flutter run [options]\n'
+      '    Run your Flutter application on an attached device\n'
+      '    or in an emulator.',
   ) {
     argParser.addFlag('verbose',
         abbr: 'v',