docs(showcase): add Simple MP (#1932)

Co-authored-by: winston <hey@winston.sh>

README.md

@@ -1182,6 +1182,7 @@ If you're making an application or tool using our palette, please let us know by
 - [flotes.app](https://flotes.app/) - A free note-taking app enhanced with flashcard features
 - [AnuPpuccin](https://github.com/AnubisNekhet/AnuPpuccin) - Highly customizable theme for [Obsidian](https://obsidian.md)
 - [faerber](https://farbenfroh.io/faerber) - Website for applying custom color schemes to any wallpaper
+- [Simple MP](https://github.com/lighttigerXIV/SimpleMP-Compose) - A simple music player based on Material You design
 <!-- AUTOGEN:SHOWCASE END -->
 
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