docs: add security escalation policy (#40391)

Author: UlisesGascon

SECURITY.md

@@ -11,3 +11,9 @@ If you have discovered a security vulnerability in this project, please report i
 Please disclose it at [security advisory](https://github.com/ampproject/amphtml/security/advisories/new).
 
 This project is maintained by a team on a best effort basis. As such, vulnerability reports will be investigated and fixed or disclosed as soon as possible.
+
+## Escalation
+
+If you do not receive an acknowledgement of your report within 6 business days, or if you cannot find a private security contact for the project, you may escalate to the OpenJS Foundation CNA at `[email protected]`.
+
+If the project acknowledges your report but does not provide any further response or engagement within 14 days, escalation is also appropriate.