docs: add instructions for authenticating for system tests (#872)

* docs: add instructions for authenticating for system tests

* docs: add phrasing change to README

synthtool/gcp/templates/node_library/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].{% if metadata['repo']['requires_billing'] %}
+1.  [Enable billing for your project][billing].{% endif %} {% if metadata['repo']['api_id'] %}
+1.  [Enable the {{ metadata['repo']['name_pretty'] }} API][enable_api]. {% endif %}
+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
+{% if metadata['repo']['api_id'] %}[enable_api]: https://console.cloud.google.com/flows/enableapi?apiid={{ metadata['repo']['api_id'] }}{% endif %}
+[auth]: https://cloud.google.com/docs/authentication/getting-started

synthtool/gcp/templates/node_library/README.md

@@ -75,8 +75,7 @@ Google APIs Client Libraries, in [Client Libraries Explained][explained].
 {% if metadata['samples']|length %}
 ## Samples
 
-Samples are in the [`samples/`](https://github.com/{{  metadata['repo']['repo'] }}/tree/master/samples) directory. The samples' `README.md`
-has instructions for running the samples.
+Samples are in the [`samples/`](https://github.com/{{  metadata['repo']['repo'] }}/tree/master/samples) directory. Each sample's `README.md` has instructions for running its sample.
 
 | Sample                      | Source Code                       | Try it |
 | --------------------------- | --------------------------------- | ------ |