Add some docs about groups
Add some docs about groups

yonglehou/skydns,AdoHe/skydns,skynetservices/skydns,vinhhrv/skydns,netf/skydns,weekface/skydns,andrewrothstein/skydns,hw-qiaolei/skydns,tuannvm/skydns,nicescale/skydns,tonicbupt/skydns,no2key/skydns,ZeaLoVe/skydns,zhang0137/skydns,loveshell/skydns