Add LM and MLM tasks

This way people can find it more easily :)

You can also use those tags for your other datasets

README.md

@@ -17,8 +17,12 @@ tags:
 - language modelling
 - common crawl
 - web
-task_categories: []
-task_ids: []
+task_categories:
+- text-generation
+- fill-mask
+task_ids:
+- language-modeling
+- masked-language-modeling
 ---
 
 # Dataset Card for OLM November/December 2022 Common Crawl