Add description to card metadata (#1)

- Add description to card metadata (f25b32266d94472ac71efb334fcb1ef2bd411a58)

README.md

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 title: Matthews Correlation Coefficient
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+emoji: 🤗
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 sdk: gradio
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 app_file: app.py
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-- evaluate
-- metric
+  - evaluate
+  - metric
+description: |-
+  Compute the Matthews correlation coefficient (MCC)
+
+  The Matthews correlation coefficient is used in machine learning as a
+  measure of the quality of binary and multiclass classifications. It takes
+  into account true and false positives and negatives and is generally
+  regarded as a balanced measure which can be used even if the classes are of
+  very different sizes. The MCC is in essence a correlation coefficient value
+  between -1 and +1. A coefficient of +1 represents a perfect prediction, 0
+  an average random prediction and -1 an inverse prediction.  The statistic
+  is also known as the phi coefficient. [source: Wikipedia]
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-
 # Metric Card for Matthews Correlation Coefficient
 
 ## Metric Description