[gi-docgen] Fix up the licensing for the release checklist

[Emmanuele Bassi <ebassi src gnome org>]

commit 6fd01161dddb48be554647761c716ae540df8ced
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Mon Aug 16 14:02:55 2021 +0100

    Fix up the licensing for the release checklist

 docs/RELEASING.md | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/docs/RELEASING.md b/docs/RELEASING.md
index 478ac1b..7985ea6 100644
--- a/docs/RELEASING.md
+++ b/docs/RELEASING.md
@@ -1,3 +1,9 @@
+<!--
+SPDX-FileCopyrightText: 2021 GNOME Foundation
+
+SPDX-License-Identifier: Apache-2.0 OR GPL-3.0-or-later
+-->
+
 Release check list
 ==================