Add the CrossVersion.for3Use2_13 modifier to the readme instructions.

[sjrd]

So that Scala 3 users can use it without having to figure it out themselves.

[sjrd]

• Should we publish this without Scala versioning suffixes?

Ideally, yes. But we cannot do that as long as parts of the javalib rely on the Scala stdlib. I still have a project of getting javalib entirely rid of Scala dependencies (like we do for javalanglib). At that point we could publish a single scalajs-javalib.jar without cross-version suffix at all, and then this library, and others with a similar design, would be able to do the same.

• If the answer is no (linking to the stdlib comes to mind), then maybe relying on for3Use2_13 "forever" is not the best policy?

Hopefully, we manage to the above before there is a version of Scala that breaks bincompat with 2.13. Until then, relying on for3Use2_13 is the correct thing to do.