Specify which versions of Yarn are supported

[fulldecent]

Please specify in the README if we can use this for Yarn 4 today or if this only applies to Yarn 6 of the future.

[fulldecent]

Also, we welcome Yarn maintainers to officially recommend how to install Yarn (4+) in GitHub Actions.

Best practice in the community is to install Node.js twice.

Official guidance is published from GitHub but I can't reproduce it.