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.
this is extremely confusing, the whole point of having Intl in your app is because you will never have to have a string in your templates that is already translated. The value that you pass into intlMessage helper should be a value coming from the context.