chore: update CONTRIBUTING.md (GoogleCloudPlatform#1666)

Author: bshaffer

CONTRIBUTING.md

@@ -53,20 +53,24 @@ composer install
 ```
 
 ### Environment variables
-Set up [application default credentials](https://cloud.google.com/docs/authentication/getting-started)
-by setting the environment variable `GOOGLE_APPLICATION_CREDENTIALS` to the path to a service
-account key JSON file.
+Some tests require specific environment variables to run. PHPUnit will skip the tests
+if these environment variables are not found. Run `phpunit -v` for a message detailing
+which environment variables are missing. Then you can set those environent variables
+to run against any sample project as follows:
 
-Then set any environment variables needed by the test. Check the
-`$SAMPLES_DIRECTORY/test` directory to see what specific variables are needed.
 ```
 export GOOGLE_PROJECT_ID=YOUR_PROJECT_ID
 export GOOGLE_STORAGE_BUCKET=YOUR_BUCKET
 ```
 
+If you have access to the Google Cloud Kokoro project, decrypt the
+`.kokoro/secrets.sh.enc` file and load those environment variables. Follow
+the instructions in [.kokoro/secrets-example.sh](.kokoro/secrets-example.sh).
+
 If your tests require new environment variables, you can set them up in
-[.kokoro/secrets.sh.enc](.kokoro/secrets.sh.enc). For instructions on managing those variables,
-view [.kokoro/secrets-example.sh](.kokoro/secrets-example.sh) for more information.
+`.kokoro/secrets.sh.enc` so they pass on Kokoro. For instructions on managing those
+variables, view [.kokoro/secrets-example.sh](.kokoro/secrets-example.sh) for more
+information.
 
 ### Run the tests