RFR: Update Readme with macOs specifics [v2]

[Maurizio Cimadamore]

On Tue, 29 Mar 2022 10:45:02 GMT, Brice Dutheil <duke at openjdk.java.net> wrote:

>> that version of jtreg should be ok
>
> I hesitated to add this link since I don't if this ok to link to individual builds, even if this is Aleksey Shipilëv.

Sorry, I think it's better not to have the link (I should have made that clearer).

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PR: https://git.openjdk.java.net/jextract/pull/10