Add a code of conduct
# What does this PR do?

This PR adds a code of conduct to the project, inspired by the [Contributor Covenant](https://www.contributor-covenant.org/). To make it clearly visible it also adds:
- a badge that displays under Transformers on the main README.
- a link in the contributing guide.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7433?src=pr&el=h1) Report
> Merging [#7433](https://codecov.io/gh/huggingface/transformers/pull/7433?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a1a8ffa5126ced93c12dfb677cbe3a069f48dcf3?el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7433/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7433?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7433      +/-   ##
==========================================
- Coverage   76.85%   76.84%   -0.02%     
==========================================
  Files         181      181              
  Lines       35800    35800              
==========================================
- Hits        27513    27509       -4     
- Misses       8287     8291       +4     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7433?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/7433/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdDUucHk=) | `66.34% <0.00%> (-28.85%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7433/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `83.37% <0.00%> (+0.24%)` | :arrow_up: |
| [src/transformers/tokenization\_mbart.py](https://codecov.io/gh/huggingface/transformers/pull/7433/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWJhcnQucHk=) | `96.82% <0.00%> (+39.68%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/7433?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/7433?src=pr&el=footer). Last update [a1a8ffa...f8d87a2](https://codecov.io/gh/huggingface/transformers/pull/7433?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>(before merging, let's check out the thread we had internally on this) 