player-mpris-tail: document position tag

Author: nachitodev

polybar-scripts/player-mpris-tail/README.md

@@ -76,6 +76,7 @@ title         | The title of the current track
 track         | The track number of the current track
 length        | The length of the current track
 fmt-length    | The formatted length of the current track ([hours:]minutes:seconds)
+position      | The track current time position
 genre         | The genre of the current track
 disc          | The disc number of the current track
 date          | The date of the current track