Update README.me for country priority listing

Using the `priority_countries` key will only list the keys:

![bildschirmfoto vom 2014-05-04 12 55 14](https://cloud.githubusercontent.com/assets/164400/2872836/37d5ac44-d37b-11e3-998b-1bdf9147057e.png)

This might be a regression as the method's signature suggests a hash as the 3rd argument.