Update README.md

Added message about the unavailability of the data on the Girder server

Author: cpinter

README.md

@@ -1,3 +1,5 @@
 # SlicerRT test data repository
 
 ### The repository has moved to the [PerkLab Girder server](https://pocus.cs.queensu.ca/#collection/5cc8811601d4930406c40465)
+
+UPDATE: The server has been updated and all data removed, so it is not available there any more. Please use the master branch of this repository.