Add copyright header to docs/registering-users.md

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

docs/registering-users.md

@@ -1,3 +1,13 @@
+<!--
+SPDX-FileCopyrightText: 2018 - 2025 MDAD project contributors
+SPDX-FileCopyrightText: 2018 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2019 Aaron Raimist
+SPDX-FileCopyrightText: 2024 Dark Decoy
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 # Registering users
 
 This documentation page tells you how to create user accounts on your Matrix server.