RFR: Update Readme with macOs specifics

Mon Mar 28 17:04:13 UTC 2022

On Mon, 28 Mar 2022 17:00:20 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> README.md line 12:
>> 
>>> 10: 
>>> 11: ```sh
>>> 12: $ sh ./gradlew -Pjdk18_home=<jdk18_home_dir> -Pllvm_home=<libclang_dir> clean verify
>> 
>> I refered to `libclang_dir`, however I wonder if the value in the angle brackets has to be renamed as `llvm_home` as well.
>
> This has been updated in a recent PR - please try to refresh your branch

Yes, that was missed previously. I've addressed in my PR here: https://github.com/openjdk/jextract/pull/9

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