Update README to remove references to mysticmine website

[akien-mga]

Hi there,

The mysticmine.com website seems to be redirecting to this GitHub repo, so the links in the README saying that one can find more info and buy the game on mysticmine.com should probably be removed.

[MCMic]

Not sure about this since this README can be accessed outside of github (through a git clone or a packaged zip)

[akien-mga]

Well I mean the link to mysticmine.com should be removed and replaced by a link to https://github.com/koonsolo/MysticMine. Or it can be kept, but it should not mention that you can buy the game there, since you can't buy it on GitHub.

[MCMic]

Indeed the part about buying the game should be removed