Update README to include IntelliJ testing info.
Update README to include IntelliJ testing info.

Since some tests now rely on generated code, it would be helpful to track any necessary setup for working in IntelliJ, since it is a very common IDE option.

PiperOrigin-RevId: 375484374

GoogleCloudPlatform/DataflowTemplates,GoogleCloudPlatform/DataflowTemplates,GoogleCloudPlatform/DataflowTemplates,GoogleCloudPlatform/DataflowTemplates