Update README to link to NCCL2

README.md

@@ -1,3 +1,6 @@
+** IMPORTANT NOTE **
+** NCCL1 is no longer maintained/updated and has been replaced by NCCL2, available at http://developer.nvidia.com/nccl. **
+
 # NCCL
 
 Optimized primitives for collective multi-GPU communication.