Document Mill support for Spotless in Readme

[lefou]

[palantirtech]

Thanks for your interest in palantir/palantir-java-format, @lefou! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[changelog-app[bot]]

Generate changelog in changelog/@unreleased

Type (Select exactly one)

• [ ] Feature (Adding new functionality)
• [ ] Improvement (Improving existing functionality)
• [ ] Fix (Fixing an issue with existing functionality)
• [ ] Break (Creating a new major version by breaking public APIs)
• [ ] Deprecation (Removing functionality in a non-breaking way)
• [ ] Migration (Automatically moving data/functionality to a new system)

Description Document Mill support for Spotless in Readme

Check the box to generate changelog(s)

• [ ] Generate changelog entry

[lefou]

This is provides under the project license, Apache License, Version 2.0. I don't see the point in signing the CLA for an small addition of a link to a readme.