Add some more information on MacOSX installation
Add more information on how to deploy the default configuration file
and to edit it.
Comment all the options that need to be replaced with real values,
including:

* `image_user` (in `login/google` section)
* `ec2_access_key`, `ec2_secret_key` in `cloud/ec2hobbes` and
  `cloud/amazon-us-east-1` sections
* `username`, `password` and `project_name` in `cloud/hobbes` section
* `gce_client_id`, `gce_client_secret` and `gce_project_id` in
  `cloud/google` section