README: document board-specific id ranges

Follow up to discussion:
https://groups.google.com/a/chromium.org/d/topic/chromium-os-dev/hyN1XAy_d5c/discussion

BUG=None
TEST=None

Change-Id: I88076675451ed2d8cdc1b0659bc56a705f85eb97
Reviewed-on: https://chromium-review.googlesource.com/1182144
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
1 file changed
tree: 3f939536856c467e1a2d9aae5924e0bf41029912
  1. eclass/
  2. metadata/
  3. profiles/
  4. PRESUBMIT.cfg