update license.md file
I received a response from the innovation management on 08/04/2020: They need more time to investigate the license case for challengeR. They have a lot of requests currently. We have to be patient!
Recap: The initially chosen license (GPLv3) is not compatible with GPLv2 only licenses of dependencies (e.g. ggplot2, relations)
Decision: The license will be changed to GPLv2 or later (source: https://www.gnu.org/licenses/gpl-faq.html#AllCompatibility)
Source of license text: https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt
How to use GNU licenses for your own software: https://www.gnu.org/licenses/gpl-howto.html
Why licenses should be identified clearly: https://www.gnu.org/licenses/identify-licenses-clearly.html