Update README

add usage of dma-buf for GPUDirect

Author: alokprasad

README

@@ -262,6 +262,9 @@ Special feature detailed explanation in tests:
  2- export LIBRARY_PATH.
  e.g: export LIBRARY_PATH=/usr/local/cuda/lib64:$LIBRARY_PATH
  perform compilation as decribe in the begining of section 4 (GPUDirect usage).
+ 
+ To use dma-buf, along with use_cuda , use_cuda_dmabuf flag should be used.
+ e.g: ib_send_bw -d mlx5_0 --use_cuda=0 --use_cuda_dmabuf
 
  CUDA Runtime API support:
  To use the --gpu_touch option in Perftest, you must build Perftest with support for the CUDA Runtime API (libcudart).