Clarify sha matching etcd release tag

Author: liggitt

go.mod

@@ -482,7 +482,7 @@ replace (
 	github.com/xlab/handysort => github.com/xlab/handysort v0.0.0-20150421192137-fb3537ed64a1
 	github.com/xordataexchange/crypt => github.com/xordataexchange/crypt v0.0.3-0.20170626215501-b2862e3d0a77
 	go.etcd.io/bbolt => go.etcd.io/bbolt v1.3.3
-	go.etcd.io/etcd => go.etcd.io/etcd v0.0.0-20191023171146-3cf2f69b5738
+	go.etcd.io/etcd => go.etcd.io/etcd v0.0.0-20191023171146-3cf2f69b5738 // 3cf2f69b5738 is the SHA for git tag v3.4.3
 	go.mongodb.org/mongo-driver => go.mongodb.org/mongo-driver v1.1.2
 	go.opencensus.io => go.opencensus.io v0.21.0
 	go.uber.org/atomic => go.uber.org/atomic v1.3.2