[ADD] skeleton code for different scenes

src/scenes/startupScene.h

@@ -0,0 +1,15 @@
+#pragma once
+#include "scene.h"
+class StartupScene : public Scene
+{
+private:
+	int menuIndex;
+
+public:
+	virtual void start() override;
+	virtual void stop() override;
+	virtual void render() override;
+	virtual void update(GLFWwindow* window) override;
+	virtual void onKey(int key, int scancode, int action, int mods) override;
+};
+