Add initial CONTRIBUTING.md
Add initial CONTRIBUTING.md

Signed-off-by: Arnaud Porterie (icecrime) <55258e89a8b41a46066b3a9514b15a78c1a88d17@docker.com>

Noyan759/swarmkit,jlhawn/swarmkit,deljuven/swarmkit,docker/swarmkit,elifa/swarmkit,AliyunContainerService/swarmkit,cezarsa/swarmkit,cheyang/swarmkit,diogomonica/swarmkit,Noyan759/swarmkit,dperny/swarmkit-1,allencloud/swarmkit,dperny/swarmkit-1,alyyousuf7/swarmkit,sungwonh/swarmkit,runshenzhu/swarmkit,yongtang/swarmkit,allencloud/swarmkit,alyyousuf7/swarmkit,yp-engineering/swarmkit,RenaudWasTaken/swarmkit,diogomonica/swarmkit,elifa/swarmkit,runshenzhu/swarmkit,aaronlehmann/swarmkit,cheyang/swarmkit,AliyunContainerService/swarmkit,jlhawn/swarmkit,fate-grand-order/swarmkit,sungwonh/swarmkit,yongtang/swarmkit,docker/swarmkit,fate-grand-order/swarmkit,deljuven/swarmkit,cezarsa/swarmkit,vasil-yordanov/swarmkit,RenaudWasTaken/swarmkit,yp-engineering/swarmkit,vasil-yordanov/swarmkit,aaronlehmann/swarmkit