README: update running etcd section for pre-built etcd install
README: update running etcd section for pre-built etcd install

The current Running etcd section only shows how to run etcd for installation
with master branch. If user has installed a pre-built release following the
instructions on the release page, the ./bin/etcd won't work to bring up the
etcd. The Getting etcd section covers both, pre-built and master branch,
with recommendation of pre-built usage so the Running etcd section is updated
accordingly.

fix #9003

coreos/etcd,sebrandon1/etcd,dannysauer/etcd,fanminshi/etcd,fanminshi/etcd,vimalk78/etcd,joshix/etcd,mkumatag/etcd,joshix/etcd,rnaveiras/etcd,glevand/coreos--etcd,etcd-io/etcd,gyuho/etcd,heyitsanthony/etcd,coreos/etcd,xiang90/etcd,heyitsanthony/etcd,bdarnell/etcd,li-ang/etcd,mqliang/etcd,dannysauer/etcd,cockroachdb/etcd,rnaveiras/etcd,whuwxl/etcd,ajityagaty/etcd,ajityagaty/etcd,rnaveiras/etcd,sebrandon1/etcd,glevand/coreos--etcd,sinsharat/etcd,gyuho/etcd,lorneli/etcd,mkumatag/etcd,ajityagaty/etcd,lorneli/etcd,bluebreezecf/etcd,mqliang/etcd,vimalk78/etcd,cockroachdb/etcd,whuwxl/etcd,bluebreezecf/etcd,etcd-io/etcd,etcd-io/etcd,mkumatag/etcd,xiang90/etcd,bdarnell/etcd,mqliang/etcd,sinsharat/etcd,li-ang/etcd