change Python version in installation instructions

[Chris Et]

Hi,
I propose to change the following line in the installation instructions
conda create --name tf python=3
to
conda create --name tf python=3.6

Reason: At the moment, TensorFlow only support Python 3.5 and 3.6. It can easily happen that someone has Python 3.7 installed, which causes pip to be unable to find and install tensorflow.

[Hvass-Labs]

Thanks for the suggestion. I have given it some thought. This is a temporary fix until TensorFlow is built for Python 3.7 and then I would have to change it back again. Then next year when Python 3.8 comes out, I would have the same problem again. If people google the error message they will see the explanation. Maybe I will add a short note to the installation instructions.