Give users a hint to include _index in their datastructure

[googol7]

I spent quite some time to find out that my rows got to have an _index property to show the expand / collapse icon. So I think it would be very helpful for beginners if the datastructure example at the beginning of the REAMDE.md would include it:

const tree = [
  {
    _index: 0,
    id: 123,
    name: 'Demo'
  },
  {
    _index: 1,
    id: 456,
    name: 'Another',
    parent: 123
  },
  {
    _index: 2,
    id: 789,
    name: 'Yet Another',
    parent: 123
  },
  {
    _index: 3,
    id: 532,
    name: 'Foobar'
  }
];

I found out by looking at the sourcecode at https://github.com/reactabular/treetabular/blob/master/src/toggle-children.js#L7

getIndex = rowData => rowData._index, // Look for index based on convention