Add model readme for deepset/roberta-base-squad2
Adding a model readme for https://huggingface.co/deepset/roberta-base-squad2
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2797?src=pr&el=h1) Report
> Merging [#2797](https://codecov.io/gh/huggingface/transformers/pull/2797?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/92e974196fc35eb826f64808ae82d20c4380e3eb?src=pr&el=desc) will **increase** coverage by `1.08%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2797/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2797?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2797      +/-   ##
==========================================
+ Coverage   73.95%   75.03%   +1.08%     
==========================================
  Files          93       93              
  Lines       15272    15272              
==========================================
+ Hits        11295    11460     +165     
+ Misses       3977     3812     -165
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2797?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2797/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.39% <0%> (+1.32%)` | :arrow_up: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2797/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `94.27% <0%> (+2.2%)` | :arrow_up: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/2797/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `73.21% <0%> (+2.27%)` | :arrow_up: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/2797/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `95.77% <0%> (+9.85%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2797/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `89.93% <0%> (+81.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2797?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/2797?src=pr&el=footer). Last update [92e9741...ec005e3](https://codecov.io/gh/huggingface/transformers/pull/2797?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
