Adding OneStopEnglish corpus dataset
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Hi @lhoestq, thanks for the review.
I have made all the changes, PTAL! :) 
This PR adds OneStopEnglish Corpus containing texts classified into reading levels (elementary, intermediate, advance) for automatic readability assessment and text simplification. 

Link to the paper: https://www.aclweb.org/anthology/W18-0535.pdf 
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