git: openjdk/jdk-sandbox: classfile-api-branch: added readme note

[Adam Sotona]

Changeset: 1f5f0035
Author:    Adam Sotona <asotona at openjdk.org>
Date:      2023-03-09 17:51:15 +0000
URL:       https://git.openjdk.org/jdk-sandbox/commit/1f5f003544ba12dbd2fc6562d96ee4ff645586f2

added readme note

! README.md