Add the copyright header for reusing the document for the MASH project

This is preparation for the document to be copied to the MASH project at https://github.com/mother-of-all-self-hosting/ansible-role-backup_borg

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-backup-borg.md

@@ -1,3 +1,13 @@
+<!--
+SPDX-FileCopyrightText: 2022 - 2025 Nikita Chernyi
+SPDX-FileCopyrightText: 2022 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2022 MDAD project contributors
+SPDX-FileCopyrightText: 2022 Julian-Samuel Gebühr
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 # Setting up BorgBackup (optional)
 
 The playbook can install and configure [BorgBackup](https://www.borgbackup.org/) (short: Borg) with [borgmatic](https://torsion.org/borgmatic/) for you.