Add single dataset citations for TweetEval
This PR adds single data citations as per request of the original creators of the TweetEval dataset.

This is a recent email from the creator:

> Could I ask you a favor? Would you be able to add at the end of the README the citations of the single datasets as well? You can just copy our readme maybe? https://github.com/cardiffnlp/tweeteval#citing-tweeteval
(just to be sure that the creator of the single datasets also get credits when tweeteval is used)

Please let me know if this looks okay or if any changes are needed.

Thanks,
Gunjan
