Add link to published advisories

Change-Id: I2dcfa85c166f5c3c886e6f90b6c3c51ecb6497f8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/232085
Reviewed-by: Alexander Thomas <[email protected]>

Author: mit-mit

SECURITY.md

@@ -1,3 +1,8 @@
 ## Reporting vulnerabilities
 To report potential vulnerabilities, please see our security policy on
 [https://dart.dev/security](https://dart.dev/security).
+
+## Published security advisories
+
+For advisories published for the Dart SDK, see
+[security advisories](https://github.com/dart-lang/sdk/security/advisories?state=published).