README: list default values for config.ember.variant

From the README and various guides I've been reading, it wasn't clear whether `config.ember.variant` was something that needed to be set. Some guides say to specify it in every `config/environments/*` file, while others skip over the config option entirely.

Then I looked it up in the [source code](https://github.com/emberjs/ember-rails/blob/36067b4c5c8e81edc2f5b43cd7bf0459ca258dfd/lib/ember_rails.rb#L24). Figured it'd be good to clarify this in the README :cat: