I totally agree that screenshots and a proper description of the app in the README are a must-have for all foss apps, but as a developer I know that most of the times you prefer use your time to add new features to your app rather then documenting existing ones…
Personally I’ll try to add them to all my future projects but what I would suggest to everyone who use and love a foss app is to check out its README and, if needed, submit a pull request with an updated version of it with screenshot etc (You don’t need to be a developer to do that and it can be really appreciated)