Updated contributing link
Updated contributing link

Closes https://github.com/PistonDevelopers/piston/issues/303
bvssvni/piston,Potpourri/piston,getynge/piston,tempbottle/piston,KevinFrans3/piston,PistonDevelopers/piston,mitchmindtree/piston,placrosse/piston,oldmanmike/piston,caldwell/piston,PistonDevelopers/piston,axelmagn/piston,bvssvni/piston