Add SPDX identifiers to README and blacken.el

Author: tgamblin

README.md

@@ -19,3 +19,12 @@ The following options change the behavior of black when reformatting buffers.
 - `blacken-skip-string-normalization` Don't normalize string quotes or prefixes.
 - `blacken-fast-unsafe` Skips temporary sanity checks.
 - `blacken-line-length` Max line length enforced by blacken.
+
+## license
+
+`blacken.el` is free software; you can redistribute it and/or modify it
+under the terms of the GNU General Public License as published by the
+Free Software Foundation; either version 3, or (at your option) any later
+version.
+
+SPDX-License-Identifier: GPL-3.0-or-later

blacken.el

@@ -6,6 +6,7 @@
 ;; Homepage: https://github.com/proofit404/blacken
 ;; Version: 0.0.1
 ;; Package-Requires: ((emacs "25.2"))
+;; SPDX-License-Identifier: GPL-3.0-or-later
 
 ;; This file is free software; you can redistribute it and/or modify
 ;; it under the terms of the GNU General Public License as published