add an asciinema cast in readme

[phanirithvij]

add a demo showcase in readme.

[jpetrucciani]

for sure! what kind of demo would you like to see? just one of my examples and how you can use it? example of the prompting/tab completion? etc.?

There are also a number of examples I want to migrate from my main nix repo over here, eventually.

[phanirithvij]

Yes, a single showcase preferably < 2min
which features as many features mentioned in the readme as possible. (Which makes sense to show visually)