Add README file to testcases directory.
Add README file to testcases directory.

Change-Id: Ifc70ea501ea7197bf187ce9470c7b9de850f78e0
Reviewed-on: https://dart-review.googlesource.com/10040
Reviewed-by: Aske Simon Christensen <589eb3fd9a84f06d0ac46d3aa71dddcfb0814ccd@google.com>
Reviewed-by: Régis Crelier <c2bf536a10d9f2530c38e4e8da517718be8c39ce@google.com>

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