Merge pull request #507 from sterlingcrispin/patch-2

Update Installation-Windows.md correcting tensor flow version

docs/Installation-Windows.md

 </p>
 
 ### Install TensorFlow GPU
-Next, install `tensorflow-gpu` using `pip`. In an Anaconda Prompt with the Conda environment ml-agents activated, type in the following command _(make sure you are connected to the internet)_:
+Next, install `tensorflow-gpu` using `pip`. You'll need version 1.4.0 as newer versions require CUDA 9 which is not yet supported. In an Anaconda Prompt with the Conda environment ml-agents activated, type in the following command _(make sure you are connected to the internet)_:
 
 ```
 pip install tensorflow-gpu==1.4.0