Metal: document how to print MSL shaders. I consistently forget how to do this, so writing it down for myself and others. Bug: chromium:1423136 Bug: angleproject:8280 Change-Id: Ibe328f2c154380b49b7d70d810b90fc15a373cb9 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4727744 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org> Auto-Submit: Kenneth Russell <kbr@chromium.org>