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 part I don't understand is the use of `var tmpl = `. Nobody uses the templates like that, they have templates or partials in a file, why should we show the example using a local variable instead of just saying this is the template file.