Coverage remained the same at 100.0% when pulling 7f30115 on stephenplusplus:spp--2534 into 288ff5a on GoogleCloudPlatform:master.

What I ended up doing was doing a search for keyFilename throughout the readme and only saw examples using it. Authentication makes total sense but I didn't see it because I expected to see a well of text along the lines of:

To use our SDK you need to authenticate using a service account. The key you'll download from the service account panel is then passed in via the keyFilename parameter.

I think the Authentication section is pretty thorough already because it has to cover multiple forms of authentication (application default, service account JSON/PEM, credentials object), as well as other steps, like billing & enabling APIs. To add more to it would bloat it out even more.

I'll merge this as-is for now, since this should at least be an improvement. A PR would be awesome though, if you can think of a way to make this even better.