[Documentation] Prior and Next keys

[xpufx]

I don't recall seeing these keys on a keyboard before. Looking at the xgbutil/keyboard source they seem to be equivalent to Page_Up (Prior) and Page_Down (Next) . Maybe this is evident to everybody else. If not, it might be good to add these keys in parantheses in the Readme and config file.

[leukipp]

Thanks for your input. I added additional information to the config section!