Update README.md

explain column 'sentence1'.

README.md

@@ -16,6 +16,8 @@ The modifications are:
 2. Encoding in UTF-8.
 3. Remove data entries whose label value is '-'.
 4. Replace label values, 'neutral' to 1, 'entailment' to 0, and 'contradiction' to 2.
+5. Add one column 'sentence1', whose value is '前提：' + premise value + '结论：' + hypothsis value.
 
 ocnli_train_std.csv comes from train.50k.json.
+
 ocnli_test_std.csv comes from dev.json.