The LICENSE file has been updated to include GPLv2+:
https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt

The EngineConfig.java has been updated to use SPDX header:
https://events.static.linuxfound.org/sites/events/files/Introduction%20to%20SPDX-without%20graphics.pdf
This is to show how to use SPDX header in a new source code.

A GPL Cooperation Commitment file has been added:
https://gplcc.github.io/gplcc/Project/README.html