Explain the quantisation finetuning parameters

[XapaJIaMnu]

@afaji since your code was merged to master, there are some addition parameters, that nobody but you understands, could you add them to the readme here:

https://github.com/browsermt/students/blob/master/train-student/finetune/README.md

Notably, I have no idea what --quantize-optimization-steps does, and possibly some people would struggle to parse --quantize-log-based. Please, add a one line explanation