This is not intended to be a comprehensive test suite; I'm just trying to get us started by adding tests for some of the most important functionality.
This necessitated some minor refactoring. Previously, `main.ts` just directly invoked the main entrypoint function `run()`, which made it impossible to unit test the module without causing the side-effects of `run`.
As a workaround I created a new module `labeler.ts`, which just exports the functions we want to test without executing the main entrypoint, and simplified `main.ts` so that it only imports the entrypoint and executes it.
It's basically just a re-org. The diff makes it look way more complicated than it is.
- Document the release process in `CONTRIBUTING.md`
- Add development instructions to `CONTRIBUTING.md` (adapted from [download-artifact](https://github.com/actions/download-artifact))
- Add a test badge to the `README`
- Link to `CONTRIBUTING.md` in `README`
- Unify copy between `README.md` and `action.yml`
- Copy-pasta the CoC from [download-artifact](https://github.com/actions/download-artifact)
Patrick Ellis