Added contributing document
Added contributing document

duanhjlt/SnapKit,zht415/SnapKit,Rdxer/SnapKit,duanhjlt/SnapKit,zhaoguohui/SnapKit,Rdxer/SnapKit,zhaoguohui/SnapKit,SnapKit/SnapKit,HackRoy/SnapKit,SnapKit/SnapKit,zht415/SnapKit,HackRoy/SnapKit