Add section "Extensions of this benchmark" to Readme

Link in the Readme to datasets that extend the WMT24++ benchmark to additional languages (currently just one).

README.md

@@ -313,6 +313,9 @@ If you use any of the data released in our work, please cite the following paper
 }
 ```
 
+## Extensions of this benchmark
+- https://huggingface.co/datasets/ZurichNLP/wmt24pp-rm (German → 6 varieties of Romansh)
+
 ## Helpful Python Constants
 
 ```python