use GPL3 or later

Readme.md

@@ -53,3 +53,9 @@ it in the `server/` subdirectory and add the command used to run the server in `
 EMPER variants are defined in `bench/emper.py`.
 See the documentation there for information about how EMPER variants are build and
 which options are available.
+
+## License
+
+Code in this repository, if not otherwise noted, is licensed under the terms of
+the GNU GENERAL PUBLIC LICENSE Version 3 (or any later version). A copy of the
+license can be found in the LICENSE file or at [](https://www.gnu.org/licenses/gpl-3.0.txt).