RFR: Update Readme with macOs specifics [v2]

[Brice Dutheil]

On Tue, 29 Mar 2022 10:29:44 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> README.md line 54:
>> 
>>> 52: 
>>> 53: ```sh
>>> 54: $ sh ./gradlew -Pjdk18_home=<jdk18_home_dir> -Pllvm_home=<libclang_dir> -Pjtreg_home=<jtreg_home> jtreg
>> 
>> I noticed `jtreg` task also required this property to be set.
>> 
>> However I do have a question about which version of jtreg should be used, as a convenience I downloaded jtreg6 from Aleksey Shipilëv https://builds.shipilev.net/jtreg/
>
> 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.

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