add sprites example

doc/examples/sprites/lib/main.dart

@@ -0,0 +1,36 @@
+import 'dart:async';
+import 'dart:math';
+
+import 'package:flame/flame.dart';
+import 'package:flutter/material.dart';
+import 'package:flame/components/component.dart';
+import 'package:flame/game.dart';
+
+void main() async {
+  final Size size = await Flame.util.initialDimensions();
+  final game = MyGame(size);
+  runApp(game.widget);
+}
+
+class MyGame extends BaseGame {
+  MyGame(Size screenSize) {
+    size = screenSize;
+  }
+
+  @override
+  void onAttach() {
+    super.onAttach();
+
+    initSprites();
+  }
+
+  void initSprites() async {
+    final r = Random();
+    List.generate(500, (i) => SpriteComponent.square(32, 'test.png'))
+        .forEach((sprite) {
+      sprite.x = r.nextInt(size.width.toInt()).toDouble();
+      sprite.y = r.nextInt(size.height.toInt()).toDouble();
+      add(sprite);
+    });
+  }
+}