In addition I added some comments in the update script which describes
steps which have to be takes the first time you run the script on a
new machine.

R=titzer@chromium.org

Change-Id: Ib360e6fcdcb63eaf225f398eff60041b48f86b62
Reviewed-on: https://chromium-review.googlesource.com/883344Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50863}