Update README.md to include TEST_PROJECT_ID
Update README.md to include TEST_PROJECT_ID

TEST_PROJECT_ID is required during the build phase, otherwise the PubSub integration test in cmdline-pull will fail.
GoogleCloudPlatform/cloud-pubsub-samples-java,googlearchive/cloud-pubsub-samples-java,GoogleCloudPlatform/cloud-pubsub-samples-java,GoogleCloudPlatform/cloud-pubsub-samples-java,googlearchive/cloud-pubsub-samples-java,googlearchive/cloud-pubsub-samples-java