update link to v1 branch

Author: FabianIsensee

readme.md

@@ -1,6 +1,6 @@
 # Welcome to the new nnU-Net!
 
-Click [here](https://github.com/MIC-DKFZ/nnUNet/tree/master) if you were looking for the old one instead.
+Click [here](https://github.com/MIC-DKFZ/nnUNet/tree/nnunetv1) if you were looking for the old one instead.
 
 Coming from V1? Check out the [TLDR Migration Guide](documentation/tldr_migration_guide_from_v1.md). Reading the rest of the documentation is still strongly recommended ;-)