fix #508: list all --state values in CLI help
MM
2d ago+12+12+01234567890123456789-3-3-0123456789
The help for --state only mentions no_changes, incompatible and compatible, but it can also print metadata and unknown (see #508). I changed the description to take the values straight from the DiffResult enum so it doesn't go out of date again, and updated the README to match.
Didn't add a test because the cli module has no tests set up yet. Checked it by running --help on the built jar.
fix #508