Update README.md

Model plt5_base learned on poquad_v2 wich is equivalent of SQuAD_2 on polish language. As original dataset don't have empty questions available on hugging face I created copy named poquad_v2 with unlocked empty questions.

README.md

@@ -1,3 +1,7 @@
 ---
 license: apache-2.0
----
+datasets:
+- mzasada/poquad_v2
+language:
+- pl
+---