Add artifact docs for json test results

Bug: 
Change-Id: I9a0372f0eb4f57d73b8ec24ba0255b39e6b3dbf2
Reviewed-on: https://chromium-review.googlesource.com/729212
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515063}
1 file changed