Consider adding an example for firebase cloud functions

[ryanburns23]

Hello, first of all great work on the node SDK.

I followed the box-node-lambda-sample tutorial and got it working. Then firebase released the new cloud functions, our app is hosted on firebase and uses the realtime database so it makes sense to use the cloud functions too.

It's pretty straight forward and I was able to get firebase functions working by following the readme on the node SDK.. But it may be nice to have some doc's for people looking for help getting started.