message: add docs for testing utils
diff: import renderer from 'react-test-renderer'
  +/**
  + * Render the component (a React.createElement() or JSX expression)
  + * into its intermediate object representation with 'type',
  + * 'props', and 'children' keys
  + *
  + * The returned object can be matched with expect().toEqual(), e.g.
  + *
  + * ```js
  + * expect(render(<Foo />)).toEqual(render(<div foo='bar' />))
  + * ```
  + */
  export function render(component) {
  return renderer.create(component).toJSON()
  }
  +/**
  + * Get the HTML class names rendered by the component instance
  + * as an array.
  + *
  + * ```js
  + * expect(renderClasses(<div className='a b' />))
  + * .toEqual(['a', 'b'])
  + * ```
  + */
  export function renderClasses(component) {
  const {props: {className}} = render(component)
  return className ? className.trim().split(' ') : []
  