[infra] Add test matrix documentation
[infra] Add test matrix documentation

Change-Id: I71be9e3a8ea5419e16bf065c69235a9813b98886
Reviewed-on: https://dart-review.googlesource.com/36483
Reviewed-by: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>

dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk