RFR: Update Readme with macOs specifics

[Brice Dutheil]

On Mon, 28 Mar 2022 17:32:43 GMT, Jorn Vernee <jvernee at openjdk.org> 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'm also feeling a little split here. Providing many different instructions for different setups seems more likely to leave the documentation for some of them going out of date (this has been my experience with other projects). I think I'd prefer having 1 recommended way of installing LLVM, which is the LLVM release page, which we can also guarantee works.
> 
> For now though, I don't mind listing the alternatives. But I'd suggest hiding them in a spoiler block (using `<details><summary>LLVM location for alternative packages</summary>...</details>`). And I think we should spell out the recommended way to get the package (download, unzip/untar, point `libclang_dir` to extracted folder) a bit more front and center as well.

@JornVernee @mcimadamore If you're ok with the changes how should we proceed with this PR. I believe it needs sponsoring beforehand.

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

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