Usage docs update.

The usage docs were missing from handlebars-helper-intl. 
I modeled these after the [`dust-helper-intl`](https://github.com/yahoo/dust-helper-intl) usage docs.
We should give an example and not use placeholders like this. It's confusing as to whether you're suppose to use `"en-US"` or `"[en-US]"`, or some array, or something else. And as @caridy said, people will probably be referring these values from the template's rendering context.