readme: add language tag

Hi,

as part of the #Hacktoberfest [2023](https://github.com/huggingface/hub-docs/issues/986) this MR extends the dataset metadata and adds a language tag.

Happy merging!

README.md

@@ -18,6 +18,8 @@ dataset_info:
     num_examples: 100
   download_size: 116815
   dataset_size: 175483
+language:
+- en
 ---
 # Dataset Card for "c4-en-validation-mini"