RFR: Update Readme with macOs specifics [v2]

[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.

Brice Dutheil has updated the pull request incrementally with one additional commit since the last revision:

  Fix review comments

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Changes:
  - all: https://git.openjdk.java.net/jextract/pull/10/files
  - new: https://git.openjdk.java.net/jextract/pull/10/files/11c00131..c3a072a5

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=10&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=10&range=00-01

  Stats: 11 lines in 1 file changed: 9 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