Create README.md

add: data description

README.md

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+---
+license: apache-2.0
+---
+1. Testing data of each revision types is in their namesake folders.
+2. ``training_set.json`` is the training data.
+3. ``validation_set.json`` is the validation data.
+4. ``original_case_pool.json`` is the orignal case pool.
+5. In each folder:
+     1. ``articles_original.json`` is the unrevised law set.
+     2. ``articles_poisoned.json`` is the revised law set.
+     3. ``original.json`` contains unrevised cases.
+     4. ``poisoned.json`` contains revised cases.