README do not mention the requirement to set app-name on firebase elements

[TimvdLippe]

Description

All Firebase elements (apart from firebase-app) require the app-name property set to the corresponding name value of firebase-app. This is mentioned in the code-lab and you can find it in the demo note-app, but it is not clear from the documentation of the elements/README. This would clear up some initial confusion for new developers who are trying out the Firebase elements.

[mbleigh]

This should be documented better, but my suggestion is that unless you have a strong reason to do so, just don't put name on your apps at all. It will create a default app and then you don't need app-name on your elements.