Update README.md

I included information on winogrande_xs, as follows

|winogrande_xs | 160| 1267|1767|

README.md

@@ -291,6 +291,7 @@ The data fields are the same among all splits.
 |winogrande_m       | 2558|      1267|1767|
 |winogrande_s       |  640|      1267|1767|
 |winogrande_xl      |40398|      1267|1767|
+|winogrande_xs      |  160|      1267|1767|
 
 ## Dataset Creation