Add security report section in README.md (#1566)

This fix adds security report section in README.md
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

README.md

@@ -191,3 +191,10 @@ More resources can be found:
 
 Examples for deployment via systemd and other use cases can be found in the
 [deployment repository](https://github.com/coredns/deployment).
+
+## Security
+
+If you find a security vulnerability or any security related issues,
+please DO NOT file a public issue, instead send your report privately to
+`security@coredns.io`. Security reports are greatly appreciated and we
+will publicly thank you for it.