cluster-management: add ca doc

Based on questsions here: https://github.com/coreos/coreos-overlay/issues/327
Fixed.


On Mon, Jul 21, 2014 at 11:07 AM, Jonathan Boulle <notifications@github.com>
wrote:

> In cluster-management/setup/adding-certificate-authorities/index.md:
>
> > @@ -0,0 +1,18 @@
> > +---
> > +layout: docs
> > +title: Adding Certificate Authorities
> > +category: cluster_management
> > +sub_category: setting_up
> > +weight: 7
> > +---
> > +
> > +# Custom CA Authorities
>
> nit, CA Authorities = Certificate Authority Authorities
>
> —
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> <https://github.com/coreos/docs/pull/276/files#r15184935>.
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