Provide documentation for application service worker

[AKST]

There doesn't seem to be any documentation for the feature outlined in the issue #18, I mentioned it in the issue it self but I feel like tracking that as an issue may be easier if I create a seperate issue.

I've read the documentation a couple of times, but I can't seem to locate any mention of this feature, but I get the feeling there was documentation for this at some point, so if there's a git hash that can be pointed out I'd be happy to make a PR to pull it into the current documentation.

[ghost]

Here's the old documentation on this, as stopgap until I get time to write a new one:
https://github.com/DockYard/ember-service-worker/blob/v0.5.2/README.md#adding-service-worker-code-directly-to-your-app

[AKST]

Cheers, appreciate it, I guess if you plan to write the docs yourself then I might hold off on making a PR then