Add spdx short identifiers, use it as asdf :license.

data/README.org

@@ -0,0 +1,20 @@
+This directory contains license templates.
+
+Template arguments are designated with an identifier inside
+~{{}}~. Possible identifiers are:
+- ~current-year~
+- ~project-author~
+- ~project-name~
+
+* license sources
+
+- [[https://choosealicense.com/licenses/]]
+
+** [[https://spdx.org/licenses/][spdx]]
+
+- [[https://spdx.org/licenses/Apache-2.0.html][Apache-2.0]]
+- [[https://spdx.org/licenses/BSD-2-Clause.html][BSD-2-Clause]]
+- [[https://spdx.org/licenses/BSD-3-Clause.html][BSD-3-Clause]]
+- [[https://spdx.org/licenses/GPL-3.0-or-later.html][GPL-3.0-or-later]]
+- [[https://spdx.org/licenses/MIT.html][MIT]]
+- [[https://spdx.org/licenses/Unlicense.html][Unlicense]]