Add note about python -u to external id cleanup script

This makes it easier to track progress.

Change-Id: Iaa1a9f2006bead7d400e8c21931d8768bb4ec3a0

tools/gerrit-account-inconsistencies/remove-user-external-ids.py

@@ -7,6 +7,9 @@
 # Note the account_ids and emails both may be non unique depending
 # on the gerrit account situation. We iterate over each line in this
 # file one at a time to avoid problems with deduping in datastructures.
+#
+# Consider running this with python3 -u to get unbuffered stdout. This
+# will help you better track progress.
 
 import getpass
 import json