Update README.md

mention prefix usage

README.md

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 Original model: [nomic-embed-text-v1.5](https://huggingface.co/nomic-ai/nomic-embed-text-v1.5)
 
+## Usage
+
+Embedding text with `nomic-embed-text` requires task instruction prefixes at the beginning of each string. 
+
+For example, the code below shows how to use the `search_query` prefix to embed user questions, e.g. in a RAG application.
+
+To see the full set of task instructions available & how they are designed to be used, visit the model card for [nomic-embed-text-v1.5](https://huggingface.co/nomic-ai/nomic-embed-text-v1.5).
 
 ## Description