Add a section on supply chain and SBOM/SLSA mitigation

Author: evankanderson

docs/reference/security/threat-model.md

@@ -358,4 +358,16 @@ Knative Serving supports running application pods under
 if they apply, and will provide a warning if submitted pods use default (empty)
 values which would be incompatible with the `restricted` profile.
 
-**Mitigates**: user code execution, system code execution
+**Mitigates**: user code execution, system code execution
+
+### SLSA builds and provenance
+
+Knative signs both the [container images](verifying-images.md) and the
+[command-line binaries](verifying-cli.md) produced the project, along with an
+SBOM and a SLSA provenance statement describing how the contents were built.
+This reduces the risk of supply chain attacks by allowing administrators and
+users to validate that their artifacts are the same ones built by Knative, and
+validating the SLSA security guarantees with respect to ephemeral and
+reproducible builds.
+
+**Mitigates**: system code execution