Update README.md

Added default config to be "train"

README.md

@@ -9,6 +9,13 @@ tags:
 - language-modeling
 - masked-language-modeling
 pretty_name: SuperWIKI Cleaned
+configs:
+- config_name: default
+  default: true
+  data_files:
+  - split: train
+    path:
+    - "*.json.gz"
 ---