Add model_cards
Add model cards for new German language models. Paper [here](https://arxiv.org/abs/2010.10906)
I think we should also add some meta-information (see [here](https://github.com/huggingface/model_card)) to the models:

```
---
language: de
license: mit
datasets:
- wikipedia
---
```

And maybe we need to add `masked-lm` to the `tags` array, so that we can use the inference widget on the model page to do some nice masking experiments :)<|||||>Awesome collaboration btw :heart: :hugs: <|||||>> And maybe we need to add `masked-lm` to the `tags` array, so that we can use the inference widget on the model page to do some nice masking experiments :)

Shouldn't need to (in theory)<|||||> ```json
"architectures": [
    "BertForMaskedLM"
  ],
```

is currently missing in our BERT configs -> @brandenchan would it be possible that you add it :hugs: <|||||>> ```json
> "architectures": [
>    "BertForMaskedLM"
>  ],
> ```
> 
> is currently missing in our BERT configs -> @brandenchan would it be possible that you add it 🤗

@stefan-it Done! Out of interest, what's the difference between BertForMaskedLM and BertForPretraining?<|||||>If I remember correctly BertForPretraining loads a LM head and a NSP head (both heads trained during pretraining). Am I correct @LysandreJik?