[license][authors] Simplify LICENSE.md file to only contain license

Merged Timo Koch requested to merge feature/simplify-license-file into master

E.g. github doesn't automatically recognize our license file because the format is too complex. To increase machine-readability / meta-data extraction, I simplified the information

  • Remove statement duplicated from README.md
  • Move contributors to a separate file AUTHORS.md
  • Mention AUTHORS.md file in the README.md

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