Add extra DRY RUN indicator (#286)

Reviewed by: Brian Bennett <brian.bennett@joyent.com>

tools/releng/branch-repos-for-release

@@ -4,7 +4,7 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 #
-# Copyright 2020 Joyent, Inc.
+# Copyright 2021 Joyent, Inc.
 #
 
 #
@@ -199,6 +199,7 @@ else
 fi
 
 # Confirmation.
+[[ "$optDryRun" == "true" ]] && echo "NOTE: This is a DRY RUN" >&2
 if [[ "$optYes" != "true" ]]; then
     numRepos=$(echo "$repos" | wc -l | awk '{print $1}')
     echo "This will create the '$branch' branch on $numRepos repo(s) as necessary." >&2