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.
What's the motivation for setting up two CNAME records? If the cluster is created as deis.yourdomain.com, the controller is accessible only at deis.deis.yourdomain.com - this is confusing. If the example is for *.apps.mydomain.com or something, you'd need only one record. The controller would be deis.apps.mydomain.com and apps would be dancing-unicorn.apps.mydomain.com