Add info about how to play in README (or in game)

I noticed this game and took a look at the [demo](http://ajusa.github.io/Spell/). However, I couldn't figure out what the controls were nor how to win/die. (perhaps background knowledge on Magicka would help me) I eventually figured out to use `WASD` to move and `XCVB` to change some sprite at the top that sometimes shoots when clicking. I was thinking it might help to put that on the README :smile: .

I would submit a PR except I don't think my knowledge of how to play is that accurate xD.