renderExpandableRow example

Is there an example for expandable rows somewhere? I don't see anything in the README or on [the demo site](https://www.material-ui-datatables.com/), and I can't tell from the README what kind of array `rowsExpanded` should be either.

I've set `options.renderExpandableRow` to `(rowData, rowMeta) => ` (using [this package](https://github.com/capaj/react-tweet-embed)), and I get this error:

Warning: validateDOMNesting(...):

Start with the README and demo site, then trace the options.renderExpandableRow and rowsExpanded documentation or examples. Reproduce the TweetEmbed case and inspect the reported tbody DOM-nesting warning; done means an example explains the expected rowsExpanded array and expandable-row rendering no longer leaves the documented behavior unclear.

Written by the indexing model from the issue text.