minikube: add README (#7359)

plugins/minikube/README.md

@@ -0,0 +1,9 @@
+# minikube
+
+This plugin provides completion for [minikube](https://github.com/kubernetes/minikube).
+
+To use it, add `minikube` to the plugins array in your zshrc file.
+
+```
+plugins=(... minikube)
+```