Sort out OSS files like AUTHORS, CHANGELOG and CONTRIBUTING

[markbirbeck]

• Take the changelog out of the README and put it in CHANGELOG.md, which is more standard. Use the structure in CHANGELOG.md in docker-job which follows Keep A Changelog.
• Add a CONTRIBUTING file along the lines of the ones I have in other projects, like the CONTRIBUTING.md file in docker-job.
• Do likewise for AUTHORS, copying the AUTHORS file in docker-job. Add all the previous contributors to this file as well, to start it off.