Add note about empty source_directories.

Reviewed-on: https://projects.torsion.org/witten/borgmatic/pulls/358

docs/how-to/backup-your-databases.md

@@ -74,6 +74,17 @@ hooks:
 Note that you may need to use a `username` of the `postgres` superuser for
 this to work with PostgreSQL.
 
+If you would like to only backup databases and not source directories, you can
+specify an empty `source_directories` field because it is a mandatory field.
+
+```yaml
+location:
+    source_directories: []
+hooks:
+    mysql_databases:
+        - name: all
+```
+
 
 ### Configuration backups