Explain how to define custom test helpers

This update proposes a way to use Ember's built in helper registration.

I apologise if I've poorly grasped any of the concepts around modularization / ember-cli, but hopefully this is a good first stab at it.  I've tested the 'style' I've proposed and it does work, but if there's a more idiomatic way, I'd love to hear :smile: 
`exports default` here, no need for the intermediate variable.