Add documentation example in the README.md for the requirements.txt configuration

[C0rn3j]

https://github.com/pygobject/pygobject-stubs?tab=readme-ov-file#project-integration

Readme mentions using the requirements.txt file as it supports config-settings.
It could also provide an example of such configuration.

[lovetox]

I would assume the same as mentioned in the readme

--config-settings=config=Gtk3,Gdk3,Soup2

did that work for you?

[C0rn3j]

This has worked just fine, but it is not obvious:

pygobject-stubs --config-settings=config=Gtk3