add example using `app_futures.done()`

[jeanetteclark]

@mbjones here's a place to put the example code you ran so we can get it into the book later

[mbjones]

I dropped an example notebook into the examples directory:

• https://github.com/NCEAS/scalable-computing-examples/blob/main/04-pleasing/solution/parsl-done-yet.ipynb