docs: add instructions for authenticating for system tests (#420)

This PR was generated using Autosynth. 🌈

Synth log will be available here:
https://source.cloud.google.com/results/invocations/9943226b-8e9b-44fd-b025-446fc572242a/targets

- [ ] To automatically regenerate this PR, check this box.

Source-Link: googleapis/synthtool@363fe30

packages/google-cloud-bigquery-datatransfer/CONTRIBUTING.md

@@ -37,6 +37,15 @@ accept your pull requests.
 1.  Title your pull request following [Conventional Commits](https://www.conventionalcommits.org/) styling.
 1.  Submit a pull request.
 
+### Before you begin
+
+1.  [Select or create a Cloud Platform project][projects].
+1.  [Enable billing for your project][billing]. 
+1.  [Enable the Google BigQuery Data Transfer Service API][enable_api]. 
+1.  [Set up authentication with a service account][auth] so you can access the
+    API from your local workstation.
+
+
 ## Running the tests
 
 1.  [Prepare your environment for Node.js setup][setup].
@@ -51,15 +60,17 @@ accept your pull requests.
         npm test
 
         # Run sample integration tests.
-        gcloud auth application-default login
         npm run samples-test
 
         # Run all system tests.
-        gcloud auth application-default login
         npm run system-test
 
 1.  Lint (and maybe fix) any changes:
 
         npm run fix
 
 [setup]: https://cloud.google.com/nodejs/docs/setup
+[projects]: https://console.cloud.google.com/project
+[billing]: https://support.google.com/cloud/answer/6293499#enable-billing
+[enable_api]: https://console.cloud.google.com/flows/enableapi?apiid=bigquerydatatransfer.googleapis.com
+[auth]: https://cloud.google.com/docs/authentication/getting-started

packages/google-cloud-bigquery-datatransfer/README.md

@@ -100,8 +100,7 @@ quickstart();
 
 ## Samples
 
-Samples are in the [`samples/`](https://github.com/googleapis/nodejs-bigquery-data-transfer/tree/master/samples) directory. The samples' `README.md`
-has instructions for running the samples.
+Samples are in the [`samples/`](https://github.com/googleapis/nodejs-bigquery-data-transfer/tree/master/samples) directory. Each sample's `README.md` has instructions for running its sample.
 
 | Sample                      | Source Code                       | Try it |
 | --------------------------- | --------------------------------- | ------ |

packages/google-cloud-bigquery-datatransfer/synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/nodejs-bigquery-data-transfer.git",
-        "sha": "e039165e4bcf8eb56e4ef7b698171676eeb73be3"
+        "sha": "17efbb918aa8776778a68335e423174b8502b650"
       }
     },
     {
@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "15013eff642a7e7e855aed5a29e6e83c39beba2a"
+        "sha": "363fe305e9ce34a6cd53951c6ee5f997094b54ee"
       }
     }
   ],