Add screenshots of distinct features to README.md

[liilac]

Upstream considerations

This enhancement request is not relevant to upstream, as it relates specifically to demonstrating features of this fork.

Please fill out the following:

• Description:
  The README.md should include screenshots of border styles and other features distinct from swaywm. These are the principle difference from Sway.
• Use case:
  Screenshots are invaluable in demonstrating these features, allowing potential users to instantly see how they might work.

[fluix-dev]

Hm, I think I agree but I'm not sure where or how to include them nicely. Feel free to send a patch.

If you need some sample images, feel free to take mine: https://github.com/fluix-dev/dotfiles#screenshots