Update Github wiki url in README.
Update Github wiki url in README.

Resolves Issue #100.

Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,shinnok/tarsnap-gui,Tarsnap/tarsnap-gui,shinnok/tarsnap-gui,Tarsnap/tarsnap-gui,shinnok/tarsnap-gui,shinnok/tarsnap-gui,Tarsnap/tarsnap-gui,shinnok/tarsnap-gui