First: Thank you for considering making a contribution! We're glad this project has been valuable to you.
We try to make contributing simple. We believe code should be tested and documented. To make that simple, we have included mocha/chai tests for the CLI and also the function's handler that you can add tests for your changes to as appropriate. We're not adverse to adding sinon too. The documentation is relatively thin since we have the opportunity to refer to the serverless framework and artillery documentation. That said, we do our documenting of the CLI interaction in README to cover all use cases.
If you have questions, you can reach out to us in our gitter channel. We will do our best to respond quickly but do please keep in mind that we too have day jobs.
Thanks again for considering a contribution, we hope to be reviewing your code soon!