Document usage of link-to helper in inline form

The [Adding Attributes on a Link](http://emberjs.com/guides/templates/links/#toc_adding-additional-attributes-on-a-link) section of the guides uses the `link-to` helper in inline form, but everywhere else in the guide to links, the link-to helper is used in block from (`{{#link-to ...}}  link text {{/link-to}}`).

This documents the use of `link-to` as an inline helper: `{{link-to 'link text' 'routeName'}}`