Added link to how to contribute
Added link to how to contribute

See PistonDevelopers/piston#715
tempbottle/piston,bvssvni/piston,mitchmindtree/piston,PistonDevelopers/piston,placrosse/piston,PistonDevelopers/window,bvssvni/window,oldmanmike/piston,bvssvni/piston,caldwell/piston,getynge/piston,Potpourri/piston,PistonDevelopers/piston,KevinFrans3/piston,axelmagn/piston