Add a readthedocs config file since they now require it.

[mjambon]

Using the default given at https://blog.readthedocs.com/migrate-configuration-v2/

Closes #385

PR checklist

• [x] New code has tests to catch future regressions
• [ ] Documentation is up-to-date
• [x] CHANGES.md is up-to-date

[mjambon]

Merging this to see if the docs get rebuilt.