Add RetroPlayer theme to themes list

Author: Seglats

docs/advanced-usage/themes.md

@@ -23,4 +23,5 @@ For this reason, the theming heroes of the Spicetify community have created a hu
 12. [bluedrift/Spicetify-Throwback](https://github.com/bluedrift/Spicetify-Throwback)
 13. [m0squdev/dracula-spicetify-theme](https://github.com/m0squdev/dracula-spicetify-theme)
 14. [Adrien5902/SpicetifyCat](https://github.com/Adrien5902/SpicetifyCat)
-15. Insert your theme here!
+15. [RetroPlayer](https://github.com/Seglats/Spicetify-retro)
+16. Insert your theme here!