In the install readme add in a requirement for Tensorflow-probability?

In the install section of the readme please consider adding in something about having TensorFlow probability.

I have spent too much time trying to install Edward2 and its dependencies by hand. I tried using the command:
`pip install edward2[tensorflow] @ "git+https://github.com/google/edward2.git#egg=edward2"`

But on macOS Catalina with zsh as my command line I got an error with the @ sign where it said:
`ERROR: Invalid requirement: '@'`

It was only when I looked at setup.py directly that I saw tensorflow-probability was one of the requirements and required a separate command line install.

Up until this point I had Edward2 installed but none of the example code would work eg I got an error saying that ed.Normal did not exist.