added health_fact dataset 
CONTRIBUTOR
Hi @lhoestq,
Initially I tried int(-1) only in place of nan labels and missing values but I kept on getting this error ```pyarrow.lib.ArrowTypeError: Expected bytes, got a 'int' object``` maybe because I'm sending int values (-1) to objects which are string type
2020-12-01T16:17:56Z
https://github.com/huggingface/datasets/pull/953#issuecomment-736656892
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2020-12-01T16:17:56Z
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Added dataset Explainable Fact-Checking for Public Health Claims (dataset_id: health_fact)