Add more specific testing advice to Contributing.md
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2815?src=pr&el=h1) Report
> Merging [#2815](https://codecov.io/gh/huggingface/transformers/pull/2815?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/bed38d3afec99ce99ef8610337cb279a8fb25033?src=pr&el=desc) will **increase** coverage by `0.55%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2815/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2815?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2815      +/-   ##
==========================================
+ Coverage   75.02%   75.58%   +0.55%     
==========================================
  Files          93       93              
  Lines       15275    15275              
==========================================
+ Hits        11460    11545      +85     
+ Misses       3815     3730      -85
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2815?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/2815/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `56.49% <0%> (+27.59%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2815?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/2815?src=pr&el=footer). Last update [bed38d3...4ae60cb](https://codecov.io/gh/huggingface/transformers/pull/2815?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
