RFR: Update Readme with macOs specifics

[Brice Dutheil]

On Mon, 28 Mar 2022 16:32:42 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.

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.

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