Add OSCAR dataset card
Hi @lhoestq! When are you planning to release the version with this dataset?

BTW: What a huge README file :astonished: Next week ! Closing in favor of #1833
I started adding the dataset card for OSCAR !

For now it's just basic info for all the different configurations in `Dataset Structure`.
In particular the Data Splits section tells how may samples there are for each config. The Data Instances section show an example for each config, and it also shows the size in MB. Since the Data Instances section is very long the user has to click to expand the info. I was able to generate it thanks to the tools made by  @madlag and @yjernite :D

Cc @pjox could you help me with the other sections ? (Dataset Description, Dataset Creation, Considerations for Using the Data, Additional Information)
