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 <mit@google.com>
diff --git a/SECURITY.md b/SECURITY.md
new file mode 100644
index 0000000..a341d38
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+## Reporting vulnerabilities
+To report potential vulnerabilities, please see our security policy on
+[https://dart.dev/security](https://dart.dev/security).