update README

README.md

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 ***Module Card Instructions:*** 
 
 ## Measurement Description
-This metric is used to calculate the diversity of a group of sentences. It can be used to evaluate the diversity of generated responses on the testset (i.e., corpus level diversity). The original paper only used it as corpus-level while some may use it to calculate diversity of several sampled responses given on context (i.e., utterence level diversity).  However, we don't recommend to calculate Distinct on a small group as it is sensitive to sentence length and number.
+This metric is used to calculate the diversity of a group of sentences. It can be used to either evaluate the diversity of generated responses of the testset (i.e., corpus level diversity), or calculate diversity of a group of sampled responses given one context (i.e., utterence level diversity). The  [original paper](https://aclanthology.org/N16-1014) (Li et al. 2022) used it as corpus-level while some may use it as utterance-level. However, we don't recommend to calculate Distinct on a small group as it is sensitive to the sentence length and number.
 
 ## How to Use