Expand authorship of spec to all contributors

[dstansby]

Currently the zenodo record only lists one author: https://zenodo.org/records/4282107, but many others have contributed to the specification. It would be nice to recognise their contributions by expanding the authorship to anyone who's contributed to the specification.

[joshmoore]

See #175 which stalled out. Thoughts and opinions (and help) there welcome.