readme: add language tags

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

@@ -1,6 +1,10 @@
-# wiki-en-passages-20210101
-This is a processed dump of the English Wikipedia from 2021-01-01. Each page has been splitted into paragraphs as they appear in the text. Lists, tables and headlines had been removed. In total it has 38,080,804 passages.
-
-Further, each article contain meta-data on the number of languages this article exists in and on the number of views this article received over a 1 year period.
-
+---
+language:
+- en
+---
+# wiki-en-passages-20210101
+This is a processed dump of the English Wikipedia from 2021-01-01. Each page has been splitted into paragraphs as they appear in the text. Lists, tables and headlines had been removed. In total it has 38,080,804 passages.
+
+Further, each article contain meta-data on the number of languages this article exists in and on the number of views this article received over a 1 year period.
+
 The articles are sorted from most popular (most languages available, most views) to least popular.