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.

I felt the same and wondered if a different file was needed, but the details block looks good for me.

Regarding Linux (or else) I felt this was an opportunity to document these LLVM installs as well. And may improve usability.

After reading your comment I think that if the build should prefer an actual download of LLVM it might be better to write a gradle task that performs the download and extraction. It should be quite ready and it might improve the deterministic aspect as the build.

-------------

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