Adding ro sts dataset
@lhoestq thank you very much for the quick review and useful comments! 

I have tried to address them all, and a few comments that you left for ro_sts I have applied to the ro_sts_parallel as well (in read-me: fixed source_datasets, links to homepage, repository, leaderboard, thanks to me message, in ro_sts_parallel.py changed to camel case as well). In the ro_sts_parallel I have changed the order on the languages, also in the example, as you said order doesn't matter, but just to have them listed in the readme in the same order.

I have commented above on why we would like to keep them as separate datasets, hope it makes sense.

If there is anything else I should change please let me know.

Thanks again! @lhoestq I tried to adjust the ro_sts_parallel, locally when I run the tests they are passing, but somewhere it has the old name of rosts-parallel-ro-en which I am trying to change to ro_sts_parallel. I don't think I have left anything related to rosts-parallel-ro-en, but when the dataset_infos.json is regenerated it adds it. Could you please help me out, how can I fix this? Thanks in advance! Great, thanks for all your help! 
Adding [RO-STS](https://github.com/dumitrescustefan/RO-STS) dataset