Add READMEs to Tensorflow versions of CamemBERT and XLM-RoBERTa
Author: jplu
Pull request: yes

Add model cards.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2720?src=pr&el=h1) Report
> Merging [#2720](https://codecov.io/gh/huggingface/transformers/pull/2720?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/2ba147ecffa28e5a4f96eebd09dcd642117dedae?src=pr&el=desc) will **decrease** coverage by `0.26%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2720/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2720?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2720      +/-   ##
==========================================
- Coverage   74.09%   73.82%   -0.27%     
==========================================
  Files          93       93              
  Lines       15248    15248              
==========================================
- Hits        11298    11257      -41     
- Misses       3950     3991      +41
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2720?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/2720/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hdXRvLnB5) | `52.94% <0%> (-21.57%)` | :arrow_down: |
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/2720/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `68.79% <0%> (-3.33%)` | :arrow_down: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2720/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `84.87% <0%> (-0.82%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2720/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.3% <0%> (-0.52%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2720?src=pr&el=footer). Last update [2ba147e...312b0d4](https://codecov.io/gh/huggingface/transformers/pull/2720?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Thanks @jplu! By the way (b/c I saw you uploaded a README to S3), 
- we might support pushing READMEs from the S3 bucket to the repo automatically.
- we definitely will find a system for users to get merge rights on their model cards (via a GitHub bot maybe)

Yep, at first I intuitively thought that the method was the first bullet point you proposed, and then I finally saw that I had to do a PR.

Your second bullet point, I think, might be feasible with the Github Actions.