Add license information for CHANGELOG.md

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

CHANGELOG.md.license

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+SPDX-FileCopyrightText: 2018 - 2024 MDAD project contributors
+SPDX-FileCopyrightText: 2018 - 2025 Slavi Pantaleev
+SPDX-FileCopyrightText: 2018 Aaron Raimist
+SPDX-FileCopyrightText: 2019 Thomas Kuehne
+SPDX-FileCopyrightText: 2020 John Goerzen
+SPDX-FileCopyrightText: 2020 Julian Foad
+SPDX-FileCopyrightText: 2021 Agustin Ferrario
+SPDX-FileCopyrightText: 2021 Dan Arnfield
+SPDX-FileCopyrightText: 2022 Jost Alemann
+SPDX-FileCopyrightText: 2023 Felix Stupp
+SPDX-FileCopyrightText: 2023 Julian-Samuel Gebühr
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later