List the required version of TensorFlow

[michaeltandy]

Dear captain-pool,

Thank you for releasing this!

I've been trying to train my own copy of E2_ESRGAN and I've run into a few problems. This made me think I am on a newer version of tensorflow with a different API, so I hoped you could update the readme to list what version of tensorflow you've used to make this work?

In particular, I'm running tf.__version__ '2.2.0' and when I set both steps to false in stats.yaml and run python3 main.py --data_dir data_dir/ --log_dir log_dir/ --model_dir model_dir/ --phases "phase1_phase2" I first got the error TypeError: tf__experimental_run_v2() missing 1 required positional argument: 'kwargs' then after I resolved that I got the error TypeError: Variable is unhashable. Instead, use tensor.ref() as the key. and after I resolved that I got the error TypeError: tf__reduce() got multiple values for argument 'axis'

If you agree these errors arose because I'm on a different version of TF to you, please could you update the README to say the version you used? Thanks!