Update README.md

Add preprint link

Author: constantinpape

README.md

@@ -3,7 +3,7 @@
 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.