docs(readme): add preview

Author: D3SOX

README.md

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 # Spicetify Add to Queue After
 
+![Preview](preview.png)
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 This extension adds a context menu option that lets you insert tracks, albums, or playlists into your queue after a specific position, rather than just adding them to the end.
 
 If you've ever wanted to queue something but have it play right after a particular track, this extension is for you.