Add German Readme

[lebogg]

What is it?

• [ ] Bugfix (user facing)
• [ ] Feature (user facing)
• [ ] Codebase improvement (dev facing)
• [x] Meta improvement to the project (dev facing)

Description of the changes in your PR

• Add german README
• Adjust existing READMEs so that they are referenced correctly

Due diligence

• [x] I read the contribution guidelines.