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.
the wrapping is also relative, because you might pass the proper data values and it will work the same. I will say that we should be clear on this, and define this as optional, and describing how to do it manually.