Update README after RoBERTa addition
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1046?src=pr&el=h1) Report
> Merging [#1046](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1046?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/d8923270e6c497862f990a3c72e40cc1ddd01d4e?src=pr&el=desc) will **increase** coverage by `0.05%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1046/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1046?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1046      +/-   ##
==========================================
+ Coverage    79.6%   79.65%   +0.05%     
==========================================
  Files          42       42              
  Lines        6863     6863              
==========================================
+ Hits         5463     5467       +4     
+ Misses       1400     1396       -4
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1046?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/file\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1046/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvZmlsZV91dGlscy5weQ==) | `74.1% <0%> (+2.87%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1046?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/pytorch-transformers/pull/1046?src=pr&el=footer). Last update [d892327...b97b7d9](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1046?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks!