Improve README.md

[MarcAlx]

Hi Matt,

Thank you for your wonderful tool that's an unmissable checkpoint in writing a DMG/GBC emulator.

I've made this PR to add a new failure example that I've found easy to miss (misaligned footer text caused by SCX not being handled).

Then I complete the README.md by decomposing per layer the final image.

Screenshots have been made using my own emulator.

Feel free to give me some feedback.

PS As I'm not working on a GBC emulator yet, I may not be able to produce the same result for GBC.