Show how values for interpolation are passed to t

the interpolation values have to be passed as an array to this.t because of readArray in https://github.com/dockyard/ember-cli-i18n/blob/71cc781abb/addon/utils/t.js#L30. I first passed in a string, but only saw the first letter of it being interpolated. Took me a moment to figure out the reason...so I thought this info might be helpful in the readme.