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 
Ok, this is actually awesome :)

You might want to add that JavaScript translations have to be loaded using the translation function. The above just reads like "all of your app's translations will break without adding that line"