Add missing file format changelog for 3.16

Author: ffaf1

doc/file-format-changelog.rst

@@ -25,6 +25,12 @@ relative to the respective preceding *published* version.
 * License fields use identifiers from SPDX License List version
   ``3.28 2026-02-20``.
 
+``cabal-version: 3.16``
+-----------------------
+
+* License fields use identifiers from SPDX License List version
+  ``3.26 2025-06-10``.
+
 ``cabal-version: 3.14``
 -----------------------