Update cache command documentation

docs/cache.md

@@ -2,4 +2,15 @@
 
 Minikube supports caching non-minikube images using the `minikube cache` command. Images can be added to the cache by running `minikube cache add <img>`, and deleted by running `minikube cache delete <img>`.
 
-Images in the cache will be loaded on `minikube start`.
+Images in the cache will be loaded on `minikube start`. If you want to list all available cached images, you can use `minikube cache list` command to list. Below is an example of this functionality:
+
+```shell
+# cache a image into $HOME/.minikube/cache/images
+$ minikube cache add ubuntu:16.04
+$ minikube cache add redis:3
+
+# list cached images
+$ minikube cache list
+- redis:3
+- ubuntu:16.04
+```