add default_url_options to rails section in README

[rusikf]

Without it i have

ActionView::Template::Error: Missing host to link to! Please provide the :host parameter, set default_url_options[:host], or set :only_path to true

[sj26]

This isn't really relevant to mailcatcher, but is covered by the Action Mailer rails guide.

For instance, my host will be different to yours (I use pow), so a copy/paste will actually confuse things.

[rusikf]

If you use another host/port, it can be changed in default uri options and set in readme this note, but ok , if you don't want it, it will be so)