Document Google Cloud Storage artifact deployment

.travis.yml

@@ -20,6 +20,10 @@ script:
   - docker run --rm -v `pwd`:/io $DOCKER_IMAGE $PRE_CMD /io/.travis/build-linux-wheels.sh
   - ls wheelhouse/
 
+# Upload build artifacts to Google Cloud Storage for tagged releases.
+# For GCS deployment documentation, see: https://docs.travis-ci.com/user/deployment/gcs/
+# The keys are stored in the repository settings.
+# See: https://docs.travis-ci.com/user/environment-variables/#Defining-Variables-in-Repository-Settings
 deploy:
   provider: gcs
   access_key_id: "$GOOGLE_ACCESS_KEY"