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

[Adam Sotona]

Changeset: eb9e44ec
Author:    Adam Sotona <asotona at openjdk.org>
Date:      2023-03-09 17:53:47 +0000
URL:       https://git.openjdk.org/jdk-sandbox/commit/eb9e44ec924f691f62e42a6d1a53377d30038985

added readme note

! README.md