auto-generate the AUTHORS file

The original file was unmaintained since long time. This is now auto-generated from the git shortlog, at release time.

Relates #606 (closed)

Checklist

• Code modified for feature

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code