Otherwise, Virtuoso logs:

app-kaleidos-triplestore-1  | 06:46:27 Remove database file before recovery
app-kaleidos-triplestore-1  | 06:46:27 Could not restore database using prefix backup_
app-kaleidos-triplestore-1  | 06:46:27 Server exiting

Although the message should be clear to the user as to what they are required to do, mentioning it specifically in the README might be useful for anyone restoring a backup for the first time (perhaps in a more stressful setting where seeing unexpected logs is the last thing they want to see).