docs(installing/configure-dns): updated xip example and added alternative method

Corrected information about xip.io and added in alternative method for setting up DNS with AWS ELBs.
this record isn't necessary because nothing is being run on the TLD. For example, I only have a single CNAME record for my cluster and it works great because the controller is sitting on `deis.domain.com` and all the apps are running on `*.domain.com`.