[model_cards] Add new German Europeana BERT models
Hi,

this PR adds the model cards for two new BERT models for Historic German.

The cased and uncased BERT models were trained on a huge corpus: newspapers from [Europeana](http://www.europeana-newspapers.eu/). Time period of these (noisy) OCRed newspapers is 18th - 20th century.

More information can be found [here](https://github.com/dbmdz/berts) and more detailed results on downstream tasks [here](https://github.com/stefan-it/europeana-bert).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2805?src=pr&el=h1) Report
> Merging [#2805](https://codecov.io/gh/huggingface/transformers/pull/2805?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/68ccc04ee6c762183ff2b34b8b85d139f77cbf14?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2805/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2805?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2805   +/-   ##
=======================================
  Coverage   75.02%   75.02%           
=======================================
  Files          93       93           
  Lines       15275    15275           
=======================================
  Hits        11460    11460           
  Misses       3815     3815
```



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