This will allow automated notification of any vulnerabilities
reported against rapidjson. Prior commit history suggests that
this is only used for outputting test results and is therefore
unlikely to be security-critical, but we're gradually aiming to
get 100% coverage of all our dependencies and this should be an
easy one.

See
https://chromium.googlesource.com/chromium/src.git/+/master/docs/adding_to_third_party.md#add-a-readme_chromium

Bug: chromium:895969
Change-Id: Ie3f0db3fed525e81e4fa6e84fe96fe6c72575046
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2419035Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>