running-coreos: expand how to use ECS with new and existing clusters
running-coreos: expand how to use ECS with new and existing clusters

michelleN/docs,jonboulle/docs,mischief/coreos-docs,coreos/docs,wli6/docs,marineam/coreos-docs,ionave/docs,hkjn/docs-1,robszumski/docs,mjg59/docs,endocode/coreos-docs,iankronquist/docs,bq-xiao/docs,joshix/docs,skynet/docs,jdloft/docs,ksmaheshkumar/docs-2,atxwebs/docs