Update contributing guide for signed-off and DCO
Update contributing guide for signed-off and DCO

hackmdio/hackmd,jackycute/HackMD,Yukaii/hackmd,jackycute/HackMD,hackmdio/hackmd,hackmdio/hackmd,PeterDaveHello/hackmd,Yukaii/hackmd,PeterDaveHello/hackmd,jackycute/HackMD,PeterDaveHello/hackmd,Yukaii/hackmd