ownCloud 8: Added doc for addTranslations()

Adds documentation for the new mechanism for loading translations from https://github.com/owncloud/core/pull/11638

Please review @DeepDiver1975 @LukasReschke @butonic 
Here is an example bundle that is loaded as JS file: https://github.com/owncloud/core/blob/master/apps/files/l10n/fr.js
The bundle registration code is here: https://github.com/owncloud/core/blob/master/core/js/l10n.js#L68

One issue though is that we have both JS and JSON files because PHP cannot parse the "JS" file directly.
In the future we should port all apps to support async initialization to make it possible async load resources and only then startup the app. (currently app JS are just starting on `$(document).ready()`)