Update README.md

README.md

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 The dataset is from [Deep generative models of genetic variation capture the effects of mutations](https://www.nature.com/articles/s41592-018-0138-4).
 
 ### Label
-Label means mutation effect score of each protein based on deep mutation scanning experiment.
+Label means mutation fitness score (protein stability) of each protein based on deep mutation scanning experiment, 
+ranging from minus infinity to positive infinity, smaller means more stable.