Hi all,
thanks to @opus, who supported me a lot with his experience with the tankerkoenig api, I changed a bit the binding handling in order to save some superfluous api requests.
Additionally I added the README.md with a little description how to set up the binding.
If someone is curious and wants to test the binding, you can download the binding here: https://www.dropbox.com/s/5xpcmizc7gz40oh/org.openhab.binding.tankerkoenig-2.0.0-SNAPSHOT.jar?dl=0
I’d recommend to set it up in a development environment, since there will be a few bugs for sure. One known issue I’m already working on is, that the binding seems to update values only after a restart 
Nevertheless, I would appreciate to get it tested 