RFR: Update Readme with macOs specifics [v2]

[Brice Dutheil]

On Tue, 29 Mar 2022 10:10:33 GMT, Brice Dutheil <duke at openjdk.java.net> wrote:

>> The macOs libclang location as well as build output differ enough to warrant specific information. In particular the use of the XCode based path to find the libclang is not an easy find. The output also makes use of jpackage with `app-image` type, but the macOs _.app_ layout different enough on macOs.
>
> Brice Dutheil has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix review comments

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/

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