message: Add missing method to README
diff: @@ -130,6 +130,9 @@ embla.next()
  ...and the methods are:
  +**`selectedIndex()`**
  +Gets the current selected slide index. First slide starts at index 0.
  +
  **`next()`**
  Goes to next item. If loop is disabled and the carousel is on the last slide this method will do nothing.
  @@ -148,11 +151,11 @@ Removes all styles applied to DOM nodes and kills all event listeners for this E
  **`on(event: string, callback: function)`**
  Subscribes to a custom Embla event by firing the passed callback. Below is a list of events you can subscribe to:
  -- **init** - Triggers after the carousel has been initialized for the first time.
  -- **destroy** - Triggers after the carousel has been destroyed.
  -- **select** - Triggers when a new target slide has been selected.
  -- **dragStart** - Triggers when carousel dragging begins.
  -- **dragEnd** - Triggers when carousel dragging ends.
  +- **`init`** - Triggers after the carousel has been initialized for the first time.
  +- **`destroy`** - Triggers after the carousel has been destroyed.
  +- **`select`** - Triggers when a new target slide has been selected.
  +- **`dragStart`** - Triggers when carousel dragging begins.
  +- **`dragEnd`** - Triggers when carousel dragging ends.
  **`off(event: string, callback: function)`**
  Ends subscription to a custom Embla event by removing the passed callback. This works for all events listed on the **on** method.
  