message: add basic testing docs
diff: @@ -26,6 +26,19 @@ Run app with: `npm run start`
  Build docs before publishing: `npm run build`
  +### Testing
  +
  +We test our components with [Jest](https://facebook.github.io/jest/) and
  +[react-test-renderer](https://reactjs.org/docs/test-renderer.html). You can run
  +the tests locally with `npm test` (or `npm t`). To run the tests as you work,
  +run Jest in watch mode with:
  +
  +```
  +npm t -- --watch
  +```
  +
  +See [`src/__tests__/example.js`](src/__tests/example.js) for an
  +example of how we're testing our components.
  ## Principles
  