Add screenshot to documentation

Co-authored-by: alexec <1142830+alexec@users.noreply.github.com>

Author: Copilot

README.md

@@ -2,6 +2,8 @@
 
 A tiny Go program that shows git diffs in the current directory and all git repositories in subdirectories.
 
+![Screenshot](screenshot.png)
+
 ## Usage
 
 ```bash
@@ -19,4 +21,3 @@ go build -o diff-server .
 ```
 
 Then open http://localhost:8080 (or your custom port) in a browser to view the diffs.
-# This is a test change