Update README.md

- Updated key bindings values as changed from "previous" to "prev"
- Mentioned that there are default Mac bindings as well.
Change README...

I think it makes more sense to have a section about the actual feature than the keybindings and how to change them. Changing default keybindings isn't a GitGutter feature but *Sublime* feature, so anyone who would like to change them can easily figure out how with a little googling. Plus I don't want to put too much README real estate devoted to this feature since it is not core GitGutter functionality. I would like to get some default linux key bindings tho.