chore: document chatterino sound license (#7206)

LICENSES/LicenseRef-fourtf-chatterino-sound-license.txt

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+Copyright (c) 2016 fourtf
+
+Permission is granted to use, copy, and distribute this sound effect:
+
+- as part of Chatterino and derivative versions of Chatterino;
+- in recordings, streams, or demonstrations of Chatterino or its derivatives, without attribution;
+- in other software, provided it is clearly identified as the "Chatterino notification sound."
+
+Any other use requires prior permission from the copyright holder.

resources/sounds/REUSE.toml

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+# SPDX-FileCopyrightText: 2026 Rasmus Karlsson <rasmus.karlsson@pajlada.com>
+# SPDX-License-Identifier: CC0-1.0
+
+version = 1
+
+[[annotations]]
+path = "ping2.wav"
+SPDX-FileCopyrightText = "Copyright (c) 2016 fourtf"
+SPDX-License-Identifier = "LicenseRef-fourtf-chatterino-sound-license"