[examples/language-modeling] Add dataset download instructions
I had to hunt for instructions to get the dataset used in this set of examples, so this PR proposes to add them to README.md.

@patrickvonplaten

Isn’t this one simply on the [HuggingFace datasets hub](https://huggingface.co/datasets) by the way?

Op wo 30 dec. 2020 om 07:50 schreef Stas Bekman <notifications@github.com>

> I had to hunt for instructions to get the dataset used in this set of
> examples, so this PR proposes to add them to README.md.
>
> @patrickvonplaten <https://github.com/patrickvonplaten>
> ------------------------------
> You can view, comment on, or merge this pull request online at:
>
>   https://github.com/huggingface/transformers/pull/9356
> Commit Summary
>
>    - [examples/language-modeling] Add dataset download instructions
>
> File Changes
>
>    - *M* examples/language-modeling/README.md
>    <https://github.com/huggingface/transformers/pull/9356/files#diff-28c51ae2110e09a5e495a1748a8ecc3c2e3cb2f7a244c000c67a9d8c4c37adf6>
>    (9)
>
> Patch Links:
>
>    - https://github.com/huggingface/transformers/pull/9356.patch
>    - https://github.com/huggingface/transformers/pull/9356.diff
>
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<|||||>Sure, let's have the equivalent instructions to retrieve that from HF `datasets`. It doesn't really matter where it comes from as long as it doesn't require the user to go and search for it.  

FWIW, I went to https://huggingface.co/datasets and:
1. couldn't find it. That is I did find `wikitext`, but how do I know that it's the same as `wikitext-2-raw-v1` that the script expects - it seems to be very specific.
2. it gives me no instructions on how to download it in the format the script expects it in.

p.s. it looks like  Email replies do not support Markdown.
<|||||>There is no need to download the data manually with the new scripts, it is done automatically by the datasets library. So this should not be added in my opinion.<|||||>oh, ok, I guess I didn't pay attention to the command line being changed and assumed that I needed to get the dataset first.

I stand corrected. Thank you for your feedback.