add description to metadata

This will add a description to the overview page: https://huggingface.co/metrics

README.md

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 - evaluate
 - metric
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+  Directional Bias Amplification is a metric that captures the amount of bias (i.e., a conditional probability) that is amplified. This metric was introduced in the ICML 2021 paper ["Directional Bias Amplification"](https://arxiv.org/abs/2102.12594)
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 # Metric Card for Directional Bias Amplification