This project should have its own git repo. The git repo should be setup as follows.
Clone the service template~[1]. IT has got most of the things you need to get started - it exposes endpoints, and it's easy to add your own; it allows you to interact with RESTBase and the MW action API in an easy manner; and it also has a load of goodies that come in handy once the service is in production: a streamlined build and deployment process, logging and metrics collection, scaling, etc. You can read a bit more about it in the documentation~[2]
[1] https://github.com/wikimedia/service-template-node
[2] https://www.mediawiki.org/wiki/Documentation/Services