Add security issues to contributing.md.

Refs #7096

CONTRIBUTING.md

@@ -71,6 +71,10 @@ Check the [cakephp-codesniffer](https://github.com/cakephp/cakephp-codesniffer)
 repository to setup the CakePHP standard. The [README](https://github.com/cakephp/cakephp-codesniffer/blob/master/README.md) contains installation info
 for the sniff and phpcs.
 
+## Reporting a Security Issue
+
+If you've found a security related issue in CakePHP, please don't open an issue in github. Instead contact us at security@cakephp.org. For more information on how we handle security issues, [see the CakePHP Security Issue Process](http://book.cakephp.org/3.0/en/contributing/tickets.html#reporting-security-issues).
+
 # Additional Resources
 
 * [CakePHP coding standards](http://book.cakephp.org/3.0/en/contributing/cakephp-coding-conventions.html)