Add screenshots section to README

Added a section for screenshots in the README.

Author: beacoder

README.md

@@ -68,6 +68,10 @@ A Python port of the Emacs [gptel-agent-harness](https://github.com/beacoder/gpt
   the file change and render it inline (red/green) in the TUI, so file
   edits are visible without leaving the app.
 
+## Screenshots
+
+![demo.png](demo.png)
+
 ## Install
 
 ```sh