A METADATA file provides more structured information about included
third-party code.

Change-Id: I58b285ac2d73139e5b3843ba33d4d923b676afb7
Reviewed-on: https://boringssl-review.googlesource.com/11042
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>