Edit getting started section to reflect the need to edit domain name in sites

[marcosthejew]

By default when DEBUG is set to False, the attribute domain_name of the first instance of the model Sites.Sites is used as HTTP_HOST when consuming DOP Express Urls. The problem is that this default value is "example.com" and it is a very confusing and time consuming task to find out why this does happen. It all could be fixed if you instructed on the "getting started" section of the docs to change it to the developer's actual domain when in a production environment.