Update README

README.md

@@ -51,7 +51,7 @@ print(next(iter(ds)))
  'thumbnail': [some binary data]
 }
 ```
-Each row is a unique super world denoted by the `world_id` created by the player denoted by the `pid`. Thumbnails are binary PNGs.
+Each row is a unique super world denoted by the `world_id` created by the player denoted by the `pid`. Thumbnails are binary PNGs. `unk1` describes the super world itself, including the world map, but its format is unknown as of now.
 
 You can also download the full dataset. Note that this will download ~13.5GB:
 ```python