message: 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
diff: @@ -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
  