Change the readme according the new version of code

The readme is out of line with the project, for example in readme it says to perform the training as follows:

`CUDA_VISIBLE_DEVICES='0,1,2,3' python -u train.py --network r100 --loss arcface --dataset emore`

but the option --loss and --dataset doesn´t exists, in fact now is --loss-type and --data-dir, and looking at the code, I realized that these options work completely differently from the previous ones.