[model_cards] Add card by @djstrong

(the current way to submit a model card to have it displayed on the website is to open a PR on the `transformers` repo itself)

Thanks for sharing!

model_cards/djstrong/bg_cs_pl_ru_cased_L-12_H-768_A-12/README.md

+Slavic BERT from https://github.com/deepmipt/Slavic-BERT-NER http://files.deeppavlov.ai/deeppavlov_data/bg_cs_pl_ru_cased_L-12_H-768_A-12.tar.gz