Add service worker README.md.

Bug: 795989
Change-Id: I1f90fa1f71f47fa3ba884814d932bffbc67a4a8d
Reviewed-on: https://chromium-review.googlesource.com/1134718
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574507}
8 files changed