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/.gitignore b/.gitignore
index e8b22c5..452b5a4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,7 @@
+# SPDX-FileCopyrightText: © 2021 Matthias Andreas Benkard <code@mail.matthias.benkard.de>
+#
+# SPDX-License-Identifier: LGPL-3.0-or-later
+
 *~
 /.idea
 /target