Add example script that generates gif shown on the README page

[GenevieveBuckley]

In the same vein as https://github.com/napari/napari-animation/issues/163, it would be nice if the script that generated the gif example shown on the main README page was included as one of the examples here

I think @alisterburt was the person who made this example, perhaps he still has the code somewhere?

[alisterburt]

agree! unfortunately this was a quick bodge and I don't have the file any more - can think about using a new gif with code or remaking