Explain how els fallbacks work

[fsmanuel]

Els always uses a plain object as a fallback (https://github.com/funkensturm/ember-local-storage/blob/master/addon/helpers/storage.js#L35) so if the app depends on localStorage being present the consuming app needs to use tryStorage and handle the undefined case on there own to inform the user that the app may be useless because nothing is persisted to localStorage