Document GitHub action outputs

[mnquintana]

Closes #293

This PR documents Chromatic's GitHub action's outputs, both in the Readme and in the action.yml manifest. It also adds a missing required field in the GitHub action schema – required.

[ghengeveld]

Thanks for contributing!

[mnquintana]

👋🏽 Just checking in – anything I else I need to do to get this merge-ready?

[ghengeveld]

All good, thanks a lot! 👍

[mnquintana]

@ghengeveld Awesome, thanks so much for giving this a look! Would you also be able to publish a new version of the action soon?

[ghengeveld]

Released in v5.7.0.

[mnquintana]

Thanks a lot @ghengeveld! Would you also be able to publish a new release with the latest changes at https://github.com/chromaui/action? It looks like the last release was v1 back in September. Not a huge deal because it's still usable by pointing my GitHub Actions config at the latest commit hash, but it would be nice to express it in terms of a semver range again.