Create SECURITY.md (#592)

* Create SECURITY.md

* Nit: formatting

* Update security report URL

Author: rictic

SECURITY.md

@@ -0,0 +1,11 @@
+# Security Policy
+
+## Reporting a Vulnerability
+
+To report a suspected vulnerability please report it through GitHub at https://github.com/webcomponents/polyfills/security/advisories/new
+
+Please do not report any suspected vulnerabilities through public issues.
+
+## Supported versions
+
+Only the latest major version of each polyfill is supported with security updates.