Add testing and documentation for startup/double #455
Description
Overview
Currently we have a mechanism for testing two clients in starlight using startup/double. However this is not included in the tests, or sufficiently in the readme.
Blocked by #384
Tasks
Describe the solution you'd like
Add a test to the zapp test using startup/double.
Add a section to the readme, explaining that functionality is available and a link to the Swaps section to demonstrate how it can be tested.
Definition of done
When is this task complete?
The two above tasks completed.