readme: add a section about signed images

Co-authored-by: Mikko Ylinen <mikko.ylinen@intel.com>
Signed-off-by: Tuomas Katila <tuomas.katila@intel.com>

Author: tkatila

README.md

@@ -31,6 +31,7 @@ Table of Contents
     * [Supported Kubernetes versions](#supported-kubernetes-versions)
     * [Release procedures](#release-procedures)
 * [Pre-built plugin images](#pre-built-plugin-images)
+    * [Signed container images](#signed-container-images)
 * [License](#license)
 * [Helm charts](#helm-charts)
 
@@ -291,6 +292,12 @@ the branches and releases in this repository.
 [imagePullPolicy](https://kubernetes.io/docs/concepts/containers/images/#updating-images)
 ```IfNotPresent``` and can be changed with ```scripts/set-image-pull-policy.sh```.
 
+### Signed container images
+
+Starting from 0.31 release, the images (`0.31.0` etc., not `devel`) are signed with keyless signing using `cosign`. The signing proof is stored in [rekor.sigstore.dev](https://rekor.sigstore.dev) in an append-only transparency log. The signature is also stored within the dockerhub.
+
+To verify the signing in Kubernetes, one can use [policy managers](https://docs.sigstore.dev/policy-controller/overview/) with [keyless authorities](https://docs.sigstore.dev/policy-controller/overview/#configuring-keyless-authorities).
+
 ## License
 
 All of the source code required to build intel-device-plugins-for-kubernetes