Improve adding_an_addon.md guide
Improve adding_an_addon.md guide

dims/minikube,kubernetes/minikube,dlorenc/minikube,warmchang/minikube,kubernetes/minikube,aaron-prindle/minikube,dlorenc/minikube,kubernetes/minikube,aaron-prindle/minikube,warmchang/minikube,dlorenc/minikube,dlorenc/minikube,dlorenc/minikube,kubernetes/minikube,kubernetes/minikube,dalehamel/minikube,kubernetes/minikube,dims/minikube,dlorenc/minikube,warmchang/minikube,r2d4/minikube,dalehamel/minikube,warmchang/minikube,warmchang/minikube,warmchang/minikube,aaron-prindle/minikube,kubernetes/minikube,dlorenc/minikube,r2d4/minikube,r2d4/minikube,warmchang/minikube,dlorenc/minikube,dalehamel/minikube,dims/minikube