Add Code Coverage and Black badges to README
Add Black badge to README based on this:
https://github.com/huggingface/transformers/blob/403d3098572ac308416653648456a940860da39e/.circleci/config.yml#L101 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4979?src=pr&el=h1) Report
> Merging [#4979](https://codecov.io/gh/huggingface/transformers/pull/4979?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/403d3098572ac308416653648456a940860da39e&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4979/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4979?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4979      +/-   ##
==========================================
- Coverage   77.20%   77.20%   -0.01%     
==========================================
  Files         128      128              
  Lines       21851    21851              
==========================================
- Hits        16870    16869       -1     
- Misses       4981     4982       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4979?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4979/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.38% <0.00%> (-0.41%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4979?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4979?src=pr&el=footer). Last update [403d309...5be7204](https://codecov.io/gh/huggingface/transformers/pull/4979?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
