Better documentation about Fabric API keys

[samtstern]

Context: #450

With Firebase and Fabric now under one roof we're going to have more fabric developers coming to FirebaseUI over time. We should document all the possible pitfalls for them in the README.md

[samtstern]

https://firebase.google.com/support/guides/digits-android

[samtstern]

I believe this is a duplicate of #904 which was fixed by #905