Fixed security notice

Author: chendrix

SECURITY.md

@@ -1 +1,5 @@
-https://github.com/StyraInc/opa-sdk-demos
+# Security Policy
+
+Please refer to the [OPA Security Policy](https://openpolicyagent.org/security)
+for details on how to report security issues, our disclosure policy, and how to
+receive notifications about security issues.