message: Add comment to indicate the protobuf version
diff: @@ -346,6 +346,7 @@ def tf_workspace(path_prefix = "", tf_repo_name = ""):
  },
  )
  + # The commit sha in the URLs correspond to v3.6.0 of protobuf
  PROTOBUF_URLS = [
  "https://mirror.bazel.build/github.com/google/protobuf/archive/v3.6.1.2.tar.gz",
  "https://github.com/google/protobuf/archive/v3.6.1.2.tar.gz",
  