Update security guidance to link to SECURITY.md (#177)

Co-authored-by: Rand McKinney <[email protected]>
Co-authored-by: Dave Kozma <[email protected]>

CONTRIBUTING.md

@@ -44,4 +44,5 @@ feel free to reach out to existing committers to have a conversation about that.
 
 ## Security Issues
 
-Security issues shouldn't be reported on this issue tracker. Instead, [file an issue to our security experts](https://helpx.adobe.com/security/alertus.html).
+Do not create a public GitHub issue for any suspected security vulnerabilities. Instead, please file an issue through [Adobe's HackerOne page](https://hackerone.com/adobe?type=team). 
+For more information on reporting security issues, see [SECURITY.md](SECURITY.md).