README: clarify etcd's use of raft
README: clarify etcd's use of raft

etcd has since started using a new raft implementation that more cleanly
separates the state machine from the log and RPC layer. Let people know
about this change.

yonglehou/raft,DreamLee471/raft,1d4Nf6/raft,alex-quiterio/raft,nporsche/raft,goraft/raft,fengshao0907/raft