Clarify licensing through REUSE annotations.

 - Manual: GFDL-1.3-or-later
 - Library: LGPL-3.0-or-later
 - Examples: GPL-3.0-or-later

Change-Id: I1d1bfede7ba51c4043f8d7e572aa88b42cc8bacf
diff --git a/jgvariant-core/src/main/java/eu/mulk/jgvariant/core/Signature.java b/jgvariant-core/src/main/java/eu/mulk/jgvariant/core/Signature.java
index c697730..d61b9d1 100644
--- a/jgvariant-core/src/main/java/eu/mulk/jgvariant/core/Signature.java
+++ b/jgvariant-core/src/main/java/eu/mulk/jgvariant/core/Signature.java
@@ -1,3 +1,7 @@
+// SPDX-FileCopyrightText: © 2021 Matthias Andreas Benkard <code@mail.matthias.benkard.de>
+//
+// SPDX-License-Identifier: LGPL-3.0-or-later
+
 package eu.mulk.jgvariant.core;
 
 import java.nio.ByteBuffer;