RFR: Update Readme with macOs specifics

[Maurizio Cimadamore]

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.

In general the changes look ok - but I wonder if this leaves things inconsistent. E.g. in the current document there's no specific description for any of the platforms involved. With your changes we go quite in a bit of details to explain how to fetch libclang for macOs users - but we leave e.g. Linux users in the dark. In general, the process of downloading an LLVM snapshot from the LLVM binary website works, I think, on every platform/OS (including macOs). The instruction you provide seem more to answer the question: what if I _already_ have some LLVM installed (which can be true with macOs with Xcode or brew, but also on Linux e.g. with apt).

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

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