Update pip install guide in README.md

Clarify that the GPU package works with Ubuntu and Windows only - same as in this pip install guide https://www.tensorflow.org/install/pip

README.md

@@ -45,7 +45,7 @@ $ pip install tensorflow
 ```
 
 Use the GPU package for
-[CUDA-enabled GPU cards](https://www.tensorflow.org/install/gpu):
+[CUDA-enabled GPU cards](https://www.tensorflow.org/install/gpu) *(Ubuntu and Windows)*:
 
 ```
 $ pip install tensorflow-gpu