os/adding-CAs: Add links to auth'd registry doc.
os/adding-CAs: Add links to auth'd registry doc.

Add links to the auth'd-registry doc, and to the `--insecure-registry`
section therein. Fixes https://github.com/coreos/docs/issues/648.

coreos/docs,jonboulle/docs,atxwebs/docs,mjg59/docs,endocode/coreos-docs,joshix/docs