docs: add quick description to setup multiple managers
docs: add quick description to setup multiple managers

Signed-off-by: Alexandre Beslic <7f00c88b7660208d6ae438e5aa377b1c70b9009e@gmail.com>

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