Add Security Vulnerabilities

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,7 @@
+<!--
+For Security Vulnerabilities, please use https://pivotal.io/security#reporting
+-->
+
 <!--
 Thanks for contributing to Spring Security. Please provide a brief description of your pull-request and reference any related issue numbers (prefix references with #).
 -->