add nccl restrict (#925)

doc/fluid/beginners_guide/install/install_CentOS.md

@@ -21,6 +21,15 @@ CentOS系统下您可以使用`cat /proc/cpuinfo | grep avx2`来检测您的处
 	* *CUDA 工具包8.0配合cuDNN v7*
 	* *GPU运算能力超过1.0的硬件设备*
 	
+* 请确保您已经正确安装nccl2，或者按照以下指令安装nccl2（这里提供的是CentOS 7，CUDA9，cuDNN7下nccl2的安装指令），更多版本的安装信息请参考NVIDIA[官方网站](https://developer.nvidia.com/nccl/nccl-download):
+
+ 
+    	wget http://developer.download.nvidia.com/compute/machine-learning/repos/rhel7/x86_64/nvidia-machine-learning-repo-rhel7-1.0.0-1.x86_64.rpm
+  		rpm -i nvidia-machine-learning-repo-rhel7-1.0.0-1.x86_64.rpm
+  		sudo apt-get install -y libnccl2=2.3.7-1+cuda9.0 libnccl-dev=2.3.7-1+cuda9.0
+  		yum update -y
+  		yum install -y libnccl-2.3.7-2+cuda9.0 libnccl-devel-2.3.7-2+cuda9.0 libnccl-static-2.3.7-2+cuda9.0
+
 
 
 ## 选择如何安装