Bootstrap Tokens should be documented on the site

[luxas]

This is a...

• Feature Request
• Bug Report

Problem:

In v1.6, we've added something called Bootstrap Tokens:

• Features issue: Integrate Cluster Bootstrap/Discovery with Kubenetes Core enhancements#130
• Proposal: A proposal for bootstrap discovery. community#189
• Initial implementation PR: BootstrapSigner and TokenCleaner controllers kubernetes#36101

There is no other documentation except for the proposal, which doesn't even cover all the topics included here

Proposed Solution:

The proposal needs updating, and a user/admin-facing doc has to be present on the site about what the fuzz is about and how to use it. Further, kubeadm will take advantage of these bootstrap tokens in v1.6 and we should explain why.

Page to Update:
http://kubernetes.io/...??

Maybe /docs/admin/{authentication,authorization}/bootstrap-tokens?

Targets v1.6

[luxas]

cc @roberthbailey @mikedanese @jbeda @dmmcquay @pires @dgoodwin @errordeveloper @devin-donnelly @deads2k @liggitt