README updated for SNLI, MNLI
Regarding the license issue, I think we should allow it since it starts with `other-`. Cc @gchhablani what do you think ? @lhoestq I agree, I'll look into it.
Closes #2275. Mentioned about -1 labels in MNLI, SNLI and how they should be removed before training. @lhoestq `check_code_quality` test might fail for MNLI as the license name `other-Open Portion of the American National Corpus`  is not a registered tag for 'licenses'