Add instruction to use specified GPU id (pytorch#73)

Related to pytorch#72 Is this information available in docs?

Author: wkentaro

mnist/README.md

@@ -3,4 +3,5 @@
 ```bash
 pip install -r requirements.txt
 python main.py
+# CUDA_VISIBLE_DEVICES=2 python main.py # to specify GPU id to ex. 2
 ```