Clarification of model upload instructions
## What changed

In the `README.md`, I specified two details in the model upload part:
- added `folder` to the `./path/to/pretrained_model/` just to make sure it is straightforward
- added a comment line specifying that the configuration file should be entitled `config.json` for the model to appear on the website. Linked with [this issue](https://github.com/huggingface/transformers/issues/4322) I raised. 

This [website page](https://huggingface.co/transformers/model_sharing.html) should also be modified accordingly but I wasn't sure where to include the changes. 

## Additional potential improvements

- One extra thing I thought of was to specify what kind of model files is accepted when mentioning the folder but I didn't have information on that. My upload with PyTorch weights went smoothly. What about TF weights? Does uploading only the three TF model files work? Is that the case for TF1 and TF2 weights?
- One minor improvement could also be to mention earlier that the model name on hugging face will be the same as the name of the uploaded folder. In my case, I read this after uploading the folder and I had to delete the files on S3 and re-upload with a new folder name that suited me better.



# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4333?src=pr&el=h1) Report
> Merging [#4333](https://codecov.io/gh/huggingface/transformers/pull/4333?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/241759101e7104192d01a07fc70432fa02ae8cb7&el=desc) will **decrease** coverage by `1.63%`.
> The diff coverage is `77.07%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4333/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4333?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4333      +/-   ##
==========================================
- Coverage   78.18%   76.55%   -1.64%     
==========================================
  Files         120      128       +8     
  Lines       20020    21502    +1482     
==========================================
+ Hits        15652    16460     +808     
- Misses       4368     5042     +674     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4333?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/commands/transformers\_cli.py](https://codecov.io/gh/huggingface/transformers/pull/4333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy90cmFuc2Zvcm1lcnNfY2xpLnB5) | `0.00% <0.00%> (ø)` | |
| [src/transformers/configuration\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/4333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2FsYmVydC5weQ==) | `100.00% <ø> (ø)` | |
| [src/transformers/configuration\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/4333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JhcnQucHk=) | `93.33% <ø> (-0.15%)` | :arrow_down: |
| [src/transformers/configuration\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/4333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JlcnQucHk=) | `100.00% <ø> (ø)` | |
| [src/transformers/configuration\_camembert.py](https://codecov.io/gh/huggingface/transformers/pull/4333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2NhbWVtYmVydC5weQ==) | `100.00% <ø> (ø)` | |
| [src/transformers/configuration\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/4333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2Rpc3RpbGJlcnQucHk=) | `100.00% <ø> (ø)` | |
| [src/transformers/configuration\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/4333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2VsZWN0cmEucHk=) | `100.00% <ø> (ø)` | |
| [src/transformers/configuration\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/4333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2VuY29kZXJfZGVjb2Rlci5weQ==) | `35.71% <0.00%> (-64.29%)` | :arrow_down: |
| [src/transformers/configuration\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/4333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2ZsYXViZXJ0LnB5) | `100.00% <ø> (ø)` | |
| [src/transformers/configuration\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/4333/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2dwdDIucHk=) | `97.22% <ø> (-0.08%)` | :arrow_down: |
| ... and [152 more](https://codecov.io/gh/huggingface/transformers/pull/4333/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4333?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4333?src=pr&el=footer). Last update [2417591...cca51b7](https://codecov.io/gh/huggingface/transformers/pull/4333?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
