Update GN docs.

The README file is two years out of date. This removes the transition status
and references to GYP, and just retains the links to related documentation.

Fixes a help typo.

Change-Id: Ie783f32e90eb24643ba06698ddc0b3206116c724
Reviewed-on: https://chromium-review.googlesource.com/949769
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#540994}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 2caa6c11ddb31646cabb905abd9b85785058b827