README: document v2 changes

v2 annotations: dataset_v2_full_with_splits.csv

v2 annotations: crossclub_500_dual_annotated.csv

v2 annotations: test_sample_500_human_verified.csv

v2 annotations: expert_adjudication_1606.xlsx

v2 annotations: human_audit_annotations.csv

v2: expert-adjudicated labels, label_source flag, frozen splits

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