readme: add disclaimer regarding CoreOS-slim
readme: add disclaimer regarding CoreOS-slim

r2d4/minikube,warmchang/minikube,aaron-prindle/minikube,warmchang/minikube,dlorenc/minikube,dlorenc/minikube,mikegrass/minikube,dongsupark/minikube,dalehamel/minikube,aaron-prindle/minikube,abbytiz/minikube,mikegrass/minikube,skipjaq/minikube,dlorenc/minikube,dlorenc/minikube,cu12/minikube,abbytiz/minikube,kubernetes/minikube,warmchang/minikube,kubernetes/minikube,cu12/minikube,warmchang/minikube,warmchang/minikube,r2d4/minikube,r2d4/minikube,dlorenc/minikube,dlorenc/minikube,dongsupark/minikube,dongsupark/minikube,endocode/minikube,dims/minikube,dlorenc/minikube,warmchang/minikube,dalehamel/minikube,kubernetes/minikube,skipjaq/minikube,kubernetes/minikube,abbytiz/minikube,mikegrass/minikube,dlorenc/minikube,endocode/minikube,cu12/minikube,warmchang/minikube,dalehamel/minikube,skipjaq/minikube,kubernetes/minikube,aaron-prindle/minikube,dims/minikube,kubernetes/minikube,dims/minikube,endocode/minikube,kubernetes/minikube