add README with explicit single train split

README.md

@@ -0,0 +1,18 @@
+---
+configs:
+- config_name: default
+  data_files:
+  - split: train
+    path: "*.parquet"
+---
+
+# test91-data
+
+LC0 self-play training shards (rescored, run `test91`).
+
+- **Rows:** 1,372,245,220
+- **Files:** 364 parquet shards
+- **Source:** raw LC0 rescored data, one shard per hour of generation.
+- **Columns:** `fen`, `played_move`, `best_move`, `is_black`, `visits`, `input_format`,
+  `rule50`, `invariance`, `dep_result`, `root_q/d/m`, `best_q/d/m`, `played_q/d/m`,
+  `orig_q/d/m`, `plies_left`, `result_q/d`, `win/draw/loss`, `game_id`, `ply`.