Use asciinema in readme to show how the levels work

Thanks to @Byron for the suggestion. Here's the link to the original comment and to aciinema

https://github.com/gitpython-developers/GitPython/pull/951
https://asciinema.org/

It would be great to show users how to use Git Gud before they download it, that way it comes off as more accessible and people know what they're getting into.