endpoints: add instructions for creating API key and service account key JSON
endpoints: add instructions for creating API key and service account key JSON


GoogleCloudPlatform/golang-samples,adamyanliev/golang-samples,adamyanliev/golang-samples,GoogleCloudPlatform/golang-samples,adamyanliev/golang-samples,adamyanliev/golang-samples,danicat/golang-samples,GoogleCloudPlatform/golang-samples,danicat/golang-samples,danicat/golang-samples