[canvaskit] Add MacOS specific notes for setup
[canvaskit] Add MacOS specific notes for setup

Change-Id: Ie42f6c7c980f952bb18b4444716cbe4681507e9f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/294601
Reviewed-by: Leon Scroggins <995cad867ca30d6399abe011583a53547c8afea1@google.com>

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