Update step-by-step guide for windows
Hi @thomwolf, for simplification purposes, I think you could remove the "`pip install ...`" steps from this commit, 'cause these deps (black, isort, flake8) are already installed on `pip install -e ".[dev]"` on the [Start by preparing your environment](https://github.com/huggingface/datasets/blob/704107f924e74445f6f0fbde69a218b72178b588/ADD_NEW_DATASET.md#start-by-preparing-your-environment)

Update step-by-step guide for windows to give an alternative to `make style`.