doc: Add a screenshot

Author: KSXGitHub

README.md

@@ -31,6 +31,8 @@ The benchmark was generated by [a GitHub Workflow](https://github.com/KSXGitHub/
 
 ## Demo
 
+![screenshot](https://user-images.githubusercontent.com/11488886/120057336-69dca180-c06c-11eb-8233-9bd4cf9cf001.png)
+
 [![asciicast of pdu command](https://asciinema.org/a/416663.svg)](https://asciinema.org/a/416663)
 
 [![asciicast of pdu command on /usr](https://asciinema.org/a/416664.svg)](https://asciinema.org/a/416664)