Add instructions on how to run the example

[prestoncarman]

I am new to Flutter and would like to test out your example, but I am not sure how to run your example.

I wanted to see how to build a table with expandable rows. The detailed and through example includes a lot of code that takes time to decipher. It would be nice to highlight just the expandable row feature.

• Add instructions on how to run example on example/README.md
• Add simple example in example/README.md that shows an expandable row.