Added example in the component system.

components/enemy.cpp

@@ -20,6 +20,10 @@ static shared_ptr<Entity> create_bullet(Entity &from) {
     // Remove them when they are outside of the screen.
     e->add(std::make_shared<Remove_Outside>(20.0f));
 
+    // Add a shield, just to show that it is possible.
+    // e->add(std::make_shared<Shield>(Tag::enemy));
+    // e->add(std::make_shared<Shield>(Tag::player));
+
     return e;
 }