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 
In the past calling the JS function `t(appName, sometext)` would make a **sync** ajax call the first time, which locked the UI. Now `t()` doesn't load any more but relies on pre-loaded bundles.