Clarify leftout split description

README.md

@@ -29,4 +29,5 @@ All three files share the same 106-column schema, including:
 - `cell_line`
 - engineered thermodynamic features
 
-`leftout` is a second held-out test set rather than a validation split.
+`leftout` is a second held-out test set containing cell lines that do not
+appear in the train or test splits.