cluster-management: add ca doc

Based on questsions here: https://github.com/coreos/coreos-overlay/issues/327
What if we restructured this paragraph to list the reasons first, then have it lead into the steps:

CoreOS supports custom Certificate Authorities (CAs) in addition to the default list of trusted CAs. Adding your own CA allows you to:

 - Use a corporate wildcard certificate
 - Use your own CA to communicate with an installation of CoreUpdate
 - Use your own CA to communicate with a private docker registry

The setup process for any of these use-cases is the same:

1. Drop the certificate authority PEM file into `/etc/ssl/certs`
2. Run the `update-ca-certificates` script