Added rust-sdl2_mixer docs
Added rust-sdl2_mixer docs

Closes https://github.com/PistonDevelopers/piston/issues/276
bvssvni/piston,PistonDevelopers/piston,oldmanmike/piston,placrosse/piston,caldwell/piston,mitchmindtree/piston,tempbottle/piston,axelmagn/piston,bvssvni/piston,Potpourri/piston,PistonDevelopers/piston,KevinFrans3/piston,getynge/piston