Update README with name option

[I just found out](https://github.com/intridea/omniauth/issues/581) that you can change the name of an omniauth strategy using the `name` option. And while this option is possible due to the Omniauth gem's behaviour and not this gem's, I think it should be documented here.

With other omniauth gems, most people will be satisfied with their URLs but in this gem's case the URL could look a bit better. So I think that the README should show that there's an option to change it.