Documenting on preview_domain in config

[svignara]

Adding the need to set the preview_domain in config as well, since this is required for the Content Preview API to be called by the application. Otherwise this conditional will always default to false for use_preview_api: https://github.com/contentful/contentful_rails/blob/e8078156d4d81024be297ca62baa4a72971c14e8/lib/contentful_rails/preview.rb#L22