nit(readme): brief description

Signed-off-by: Cameron Smith <[email protected]>

README.md

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+Pyro-Velocity is a Bayesian, generative, and multivariate RNA velocity model to estimate uncertainty in predictions of future cell states using minimal models approximating transcription and splicing dynamics applied to multi-modal single-cell transcriptomics data.
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 ## Documentation 📒
 
 Please see the [Documentation](https://docs.pyrovelocity.net).