Please describe how to use BERT or ELECTRA Models for Longformer training.

Hi,
I am interested in traing a german longformer model. But there is no german RoBERTa. I have seen that it is possible to use BERT as a basis but you only have a description to use RoBERTa. Since there seems to be some demand:

1. https://github.com/allenai/longformer/issues/85
2. https://github.com/allenai/longformer/issues/21
3. https://github.com/allenai/longformer/issues/82
4. https://github.com/allenai/longformer/issues/77
5. https://github.com/allenai/longformer/issues/72
6. https://github.com/allenai/longformer/issues/65
7. https://github.com/allenai/longformer/issues/61

Could you please add a 2nd jupyter notebook on how to use BERT as basis or describe in detail what to change on the existing notebook for RoBERTa? That would realy be awsome.

Thanks
Philip