Added the security policy to the contact page

The contact.md page includes details on how to reach out to the Tekton
community. It should include details about reporting security related
issues as well.

contact.md

@@ -30,6 +30,8 @@ group, so please join if you are interested in accessing those docs.
 Our mailing list for end users of Tekton is
 [tekton-users@](https://groups.google.com/forum/#!forum/tekton-users).
 
+To report or enquiry about security issues, please refer to our [security policy](https://github.com/tektoncd/community/security/policy).
+
 ## Calendar
 
 Tekton events such as [working group](./working-groups.md) and other meetings