Add back the noop entry hint for "becomeLeader" (#148)

from master to course branch
the noop entry is important hint for implementation, else learner will feel confused when test fail.

raft/raft.go

@@ -196,6 +196,7 @@ func (r *Raft) becomeCandidate() {
 // becomeLeader transform this peer's state to leader
 func (r *Raft) becomeLeader() {
 	// Your Code Here (2A).
+	// NOTE: Leader should propose a noop entry on its term
 }
 
 // Step the entrance of handle message, see `MessageType`