add gem source to README
add gem source to README

hannahbot/bullet,flyerhzm/bullet,jay3126/bullet,yakovenkodenis/bullet,sho-h/brakefast,flyerhzm/bullet,jay3126/bullet,rrosenblum/bullet,eileencodes/bullet,danp60/bullet,sho-h/brakefast,rstacruz/bullet,sideci-sample/sideci-sample-bullet,theass/bullet,PikachuEXE/bullet,rstacruz/bullet,dwb/bullet,dwb/bullet,eileencodes/bullet,rrosenblum/bullet,theass/bullet,yakovenkodenis/bullet,PikachuEXE/bullet,sideci-sample/sideci-sample-bullet,flyerhzm/bullet,danp60/bullet,0xCCD/bullet,hannahbot/bullet