Update README.md

README.md

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 ## Usage
 
+- Use **dot product** distance for retrieval.
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+- Use "query: " and "passage: " correspondingly for asymmetric tasks such as passage retrieval in open QA, ad-hoc information retrieval.
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+- Use "query: " prefix for symmetric tasks such as semantic similarity, bitext mining, paraphrase retrieval.
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+- Use "query: " prefix if you want to use embeddings as features, such as linear probing classification, clustering.
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 ### transformers
 
 #### Direct usage