added URL of benchmarking repo

benchmark/README

 The code written by James Bergstra in this directory has been superseded
-by the scipy benchmarking code, which is in another repository.
+by the scipy benchmarking code, which is in another repository:
+
+https://github.com/jaberg/DeepLearningBenchmarks
 
 The remainder of the code in this directory remains undocumented.