Update readme to v0.15.4 release

Author: marquiz

README.md

@@ -12,7 +12,7 @@ features and system configuration!
 #### Quick-start – the short-short version
 
 ```bash
-$ kubectl apply -k https://github.com/kubernetes-sigs/node-feature-discovery/deployment/overlays/default?ref=v0.15.3
+$ kubectl apply -k https://github.com/kubernetes-sigs/node-feature-discovery/deployment/overlays/default?ref=v0.15.4
   namespace/node-feature-discovery created
   customresourcedefinition.apiextensions.k8s.io/nodefeaturerules.nfd.k8s-sigs.io created
   customresourcedefinition.apiextensions.k8s.io/nodefeatures.nfd.k8s-sigs.io created