Document changes in scc 0.4.0

See https://github.com/openmicroscopy/snoopycrimecop/issues?milestone=5&state=closed for the set of changes included in the 0.4.0 release.
This PR should describe the principal improvement from a scc user perspective:
- addition of `scc check-status` to check the API status
- addition of `--check-commit-status` filter (currently tested with docs jobs)
- `scc unrebased-prs` improvements

Since versions 0.4.1 and 0.4.2 have been released where 
- 0.4.1 adds internal retries to deal with failing GH API calls 
- 0.4.2 makes an internal fix to hanging scc in a specific case of a Glencoe CI
Unless people really want to know more about this, I don't think there is anything to document for these 2 releases.