Make changes to german-bert vocab file more prominent
We have been approached by researchers because the expected behavior of their bert-base-german-cased models changed without code modifications. 

- So we wanted to make the changes to the vocab more prominent in the model card 
- and also support a solution where people can easily use the old version through https://huggingface.co/deepset/bert-base-german-cased-oldvocab
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4488?src=pr&el=h1) Report
> Merging [#4488](https://codecov.io/gh/huggingface/transformers/pull/4488?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/6dc52c78d8f1f96ffd9b8f8178e142b7d4a77d14&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4488/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4488?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4488      +/-   ##
==========================================
+ Coverage   78.02%   78.04%   +0.01%     
==========================================
  Files         123      123              
  Lines       20477    20477              
==========================================
+ Hits        15978    15982       +4     
+ Misses       4499     4495       -4     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4488?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4488/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.76% <0.00%> (+0.23%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4488/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.83% <0.00%> (+0.32%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4488/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4488?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/4488?src=pr&el=footer). Last update [6dc52c7...c4a85ea](https://codecov.io/gh/huggingface/transformers/pull/4488?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Great, thanks for adding this notice