show off cartridge image in readme

README.md

@@ -8,6 +8,8 @@ A port of the GTA San Andreas Arcade Game Duality.
 
 [Try it out online!](https://xythobuz.github.io/Duality)
 
+[![Cartridge](https://xythobuz.github.io/Duality/cartridge.png)](https://xythobuz.github.io/Duality)
+
 ## Getting Started
 
 You need the [GBDK-2020](https://gbdk.org/docs/api/docs_getting_started.html) to build the ROM and [Gearboy](https://github.com/drhelius/Gearboy) to emulate it comfortably.