Add link to Github repository, improve description

This PR adds a link to the github repo and improves the description to reflect the contents of the repository.

README.md

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 license: mit
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-This is the fastText pretraining data filter targeting
-the LAMBADA task, discussed in the main text of the Perplexity
-Correlations paper: https://arxiv.org/abs/2409.05816
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+This repository contains a fastText pretraining data filter targeting the LAMBADA task, as discussed in the paper [Improving Pretraining Data Using Perplexity Correlations](https://arxiv.org/abs/2409.05816). This filter selects high-quality pretraining data based on correlations between LLM perplexity and downstream benchmark performance. 
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+Code: https://github.com/TristanThrush/perplexity-correlations