Release v6.0.20

bugfixes:
- 9204b761: ...do not add to ActiveTUs when voting on a proposal

documentation:
- 9f452a62: fix link formatting in CONTRIBUTING
- 7c363797: (docker) basic usage instructions
- 27f30212: (docker) note ports and `curl` usage
- d92f1838: (docker) explain how to generate dummy data
- 1bb4daa3: merge CodingGuidelines into CONTRIBUTING.md

upgrade:
- c7c79a15: bump to v6.0.20
This tag has no release notes.