README.md: Update bare-metal docs link to matchbox
README.md: Update bare-metal docs link to matchbox

* Link to the terraform-based example which is easier for newcomers

diegs/bootkube,diegs/bootkube,kubernetes-incubator/bootkube,hongchaodeng/bootkube,dghubble/bootkube,ericchiang/bootkube,diegs/bootkube,ericchiang/bootkube,dghubble/bootkube,hongchaodeng/bootkube,kubernetes-incubator/bootkube,ericchiang/bootkube,dghubble/bootkube,hongchaodeng/bootkube,kubernetes-incubator/bootkube