Update cheatsheet.md

Document kubectl set-cluster --proxy-url

content/en/docs/reference/kubectl/cheatsheet.md

@@ -68,6 +68,11 @@ kubectl config get-contexts                          # display list of contexts
 kubectl config current-context                       # display the current-context
 kubectl config use-context my-cluster-name           # set the default context to my-cluster-name
 
+kubectl config set-cluster my-cluster-name # set a cluster entry in the kubeconfig
+
+# configure the URL to a proxy server to use for requests made by this client in the kubeconfig
+kubectl config set-cluster my-cluster-name --proxy-url=my-proxy-url
+
 # add a new user to your kubeconf that supports basic auth
 kubectl config set-credentials kubeuser/foo.kubernetes.com --username=kubeuser --password=kubepassword