docs: update guidance on service accounts

[busunkim96]

Link out to cloud.google.com docs on service accounts since it is less likely to go stale.

Fixes #946

[parthea]

LGTM

[javelinsubhash]

Not sure if this should be a new issue, but docs show two different ways of creating keys with gcloud format

{
  "type": "service_account",
  "project_id": "project-id",
  "private_key_id": "key-id",
  "private_key": "-----BEGIN PRIVATE KEY-----\nprivate-key\n-----END PRIVATE KEY-----\n",
  "client_email": "service-account-email",
  "client_id": "client-id",
  "auth_uri": "https://accounts.google.com/o/oauth2/auth",
  "token_uri": "https://accounts.google.com/o/oauth2/token",
  "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
  "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/service-account-email"
}

or REST API

{
  "name": "projects/project-id/serviceAccounts/service-account-email/keys/key-id",
  "privateKeyType": "TYPE_GOOGLE_CREDENTIALS_FILE",
  "privateKeyData": "private-key",
  "validAfterTime": "date",
  "validBeforeTime": "date",
  "keyAlgorithm": "KEY_ALG_RSA_2048"
}

For this client library are both formats valid? I'm asking because I'm having auth issues with the gcloud format for this repository