add openssf knative badge (#1088)

Signed-off-by: Carlos Santana <csantana23@gmail.com>

Author: csantanapr

SECURITY.md

@@ -3,3 +3,5 @@
 To report a security issue, please email security@knative.team with a description of the issue, the steps you took to create the issue, affected versions, and, if known, mitigations for the issue. Our vulnerability management team will respond within 3 working days of your email. If the issue is confirmed as a vulnerability, we will open a Security Advisory. This project follows a 90 day disclosure timeline.
 
 Please see [Early Disclosure of Security Vulnerabilities](https://github.com/knative/community/blob/main/working-groups/security/disclosure.md) and [Vulnerability Disclosure Response Policy](https://github.com/knative/community/blob/main/working-groups/security/responding.md) as well.
+
+Knative Open Source Security Foundation (OpenSSF) best practices badge [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/5913/badge)](https://bestpractices.coreinfrastructure.org/projects/5913)