Added creation of graphics context to `GameIterator` docs
Added creation of graphics context to `GameIterator` docs

Closes https://github.com/PistonDevelopers/piston/issues/378

placrosse/piston,tempbottle/piston,getynge/piston,bvssvni/piston,caldwell/piston,bvssvni/piston,mitchmindtree/piston,KevinFrans3/piston,PistonDevelopers/piston,oldmanmike/piston,axelmagn/piston,PistonDevelopers/piston,Potpourri/piston