RFR: Update Readme with macOs specifics

[Brice Dutheil]

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.

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Commit messages:
 - Update Readme with macOs specifics

Changes: https://git.openjdk.java.net/jextract/pull/10/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=10&range=00
  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jextract/pull/10.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/10/head:pull/10

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