Add a security policy for the Dart SDK
Add a security policy for the Dart SDK

Bug: b/200122212
Change-Id: I7f7f11f24034c525cb0f3baab3120ba15b3cb9d3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/215061
Reviewed-by: Michael Thomsen <5977a55c75bb4612004451a56b0b4c22d9d3311d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk