Explain how to use GN and say not to use GYP.

BUG=angleproject:1569

Change-Id: I972126dced28fa7a70eb301b500f172376a569c1
Reviewed-on: https://chromium-review.googlesource.com/775858Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>