docs: clarify GPL is version 2 or later in COPYING

Packagers asked whether the GPL-covered parts of Tiled are GPLv2-only
or GPLv2-or-later. Source headers use the standard FSF 'v2 or later'
wording; document this explicitly and give the SPDX id.

Fixes #2018

Made-with: Cursor

Author: PrateekSingh070

COPYING

@@ -15,3 +15,16 @@ tmxviewer-java       util/java/tmxviewer-java  BSD 2-clause license
 
 The full text of each license is provided in the files LICENSE.GPL
 and LICENSE.BSD.
+
+GPL version (2 only vs 2 or later)
+----------------------------------
+The Tiled application and its bundled plugins (see table above) are
+licensed under the GNU General Public License **version 2 or later**.
+Each source file in those directories states this in its header (the
+standard "either version 2 of the License, or (at your option) any later
+version" wording). SPDX identifier: GPL-2.0-or-later.
+
+The file LICENSE.GPL contains the complete text of GNU GPL version 2.
+The "or later" option refers to any subsequent version of the GNU GPL
+published by the Free Software Foundation, as permitted by that license
+text.