Add screenshot to documentation (#9)

* Initial plan

* Initial exploration of repository structure

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

* Add screenshot to documentation

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

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: alexec <1142830+alexec@users.noreply.github.com>
Co-authored-by: Alex Collins <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