Update Contributing Guidelines for "Building" Section

[Bubblemelon]

Under the Building section of CONTRIBUTING.md :

There should be a new link to replace "See ./vagrant/README.md for a quick workflow to build and test your own kubeadm binaries"

Since all the /vagrant files have been removed in commit: 90a02d7

As far as I can tell, based on the statement in /vagrant/README.md
and and PR #1446 , I suggest pointing to https://github.com/kubernetes/kubeadm/tree/master/kinder in the Building section of CONTRIBUTING.md.

[yagonobre]

Thanks @Bubblemelon for reporting this, would you want to send a pr with the fix?

[Bubblemelon]

You're welcome ! 😄 Checkout my pr ⬆️

[yagonobre]

Closed by #1467
/close

[k8s-ci-robot]

@yagonobre: Closing this issue.

In response to this:

Closed by #1467
/close

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