Fix eval ref miss in Chinese WWM.
Sorry for my reckless: I didn't add param for `eval_ref_file` when Chinese WWM.
It was found by @johnsonice in [here](https://github.com/huggingface/transformers/pull/7925#issuecomment-717701325).

So I fix it and update readme for Chinese WWM.
Before we merge you'll need to run `make fixup` or `make style` at the root of your transformers clone to pass the code quality test.<|||||>I had not realized that LTP has pinned master to 3.2.0. We can't have a script in examples that doesn't run on master, so I suggest copying the current version and moving it in the examples/contrib folder (or hosting it on your GitHub if you prefer) while still linking to it from the README.

We are in the process of rewriting all examples (and this script as it is will change in the next few days) to match the current version of transformers/datasets so this master requirement is really important. <|||||>> I had not realized that LTP has pinned master to 3.2.0. We can't have a script in examples that doesn't run on master, so I suggest copying the current version and moving it in the examples/contrib folder (or hosting it on your GitHub if you prefer) while still linking to it from the README.
> 
> We are in the process of rewriting all examples (and this script as it is will change in the next few days) to match the current version of transformers/datasets so this master requirement is really important.

Seem the requirements of LTP is fixed. 
So I move to `run_chinese_ref.py` to `examples/contrib` and update readme.<|||||>Great! If you can just merge Lysandre's suggestion, this should be good to merge then.<|||||>Applied the suggestion, merging!