Update README.md

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README.md

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 SynapseNet is a tool for segmentation and analysis of synapses in electron microscopy.
 
 To learn how to use SynapseNet, check out [the documentation](https://computational-cell-analytics.github.io/synapse-net/).
-To learn more about how it works, check out [our preprint](TODO).
+To learn more about how it works, check out [our preprint](https://www.biorxiv.org/content/10.1101/2024.12.02.626387v1).
 
 See an example reconstruction of a mossy fibre synapse with SynapseNet.
 Automatic segmentation of synaptic vesicles are rendered in orange, active zones in blue and two mitochondria in red and cyan.