Document service account keys
Document service account keys

Add documentation to the README for the use of service account keys for
authorization as an alternative to logging in with the gcloud cli.

Fixes #248

keymetrics/vxx,googleapis/cloud-trace-nodejs,keymetrics/trassingue,googleapis/cloud-trace-nodejs,matthewloring/cloud-trace-nodejs,matthewloring/cloud-trace-nodejs,googleapis/cloud-trace-nodejs,keymetrics/vxx,googleapis/cloud-trace-nodejs,matthewloring/cloud-trace-nodejs,keymetrics/trassingue