message: Add groupSlides to README
diff: @@ -92,6 +92,7 @@ const node = document.getElementById('embla')
  const embla = EmblaCarousel(node, {
  align: 'center',
  container: '*',
  + groupSlides: 1,
  draggable: true,
  dragFree: false,
  loop: false,
  @@ -109,6 +110,9 @@ Align the slides relative to the carousel viewport.
  **`container`** (string: querySelectorString)
  The selector to use for the container that holds the slides. Embla will use all immediate children of this node as slides.
  +**`groupSlides`** (number)
  +Groups slides together into groups of given size and makes each group act as a single slide.
  +
  **`draggable`** (boolean)
  Allow mouse and touch interactions to move the carousel.
  