Improve documentation

[mydea]

By now, the README has become quite large.
For the sake of readability, it would probably be a good idea to create an actual documentation app, and deploy it with ember-cli-github-pages.

[mydea]

@arm1n what do you think about moving the documentation into the GitHub Wiki? There, we could easily structure the sections into dedicated pages (e.g. setup, the CLI, the service, the helpers, ...).

Alternatively, I would propose to split out the giant README.md into multiple .md files.

[mydea]

@arm1n I started to port the super long README into the GitHub Wiki: https://github.com/Cropster/ember-l10n/wiki - Please let me know what you think. This should help to make this more approachable.