RFR: 7259: README.md should mention that the build script works for Mac OS

[Gunnar Morling]

On Mon, 24 May 2021 08:38:59 GMT, Gunnar Morling <github.com+28612+gunnarmorling at openjdk.org> wrote:

> Found that the build.sh script works on macOS, too.

@thegreystone That's a quick one to review ;)

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PR: https://git.openjdk.java.net/jmc/pull/251