[testing] Rename documentation file.

README.md is easier to find than message.md.

BUG=

Change-Id: I9b9b8173c322206b931176d480566cdcb62eb31c
Reviewed-on: https://chromium-review.googlesource.com/464706Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Franziska Hinkelmann <franzih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44292}