Migrate model card from transformers-repo

Read announcement at https://discuss.huggingface.co/t/announcement-all-model-cards-will-be-migrated-to-hf-co-model-repos/2755
Original file history: https://github.com/huggingface/transformers/commits/master/model_cards/allegro/herbert-klej-cased-tokenizer-v1/README.md

README.md

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+---
+language: pl
+---
+
+# HerBERT tokenizer
+
+**[HerBERT](https://en.wikipedia.org/wiki/Zbigniew_Herbert)** tokenizer is a character level byte-pair encoding with
+vocabulary size of 50k tokens. The tokenizer was trained on [Wolne Lektury](https://wolnelektury.pl/) and a publicly available subset of
+[National Corpus of Polish](http://nkjp.pl/index.php?page=14&lang=0) with [fastBPE](https://github.com/glample/fastBPE) library.
+Tokenizer utilize `XLMTokenizer` implementation from [transformers](https://github.com/huggingface/transformers).
+
+## Tokenizer usage
+Herbert tokenizer should be used together with [HerBERT model](https://huggingface.co/allegro/herbert-klej-cased-v1):
+```python
+from transformers import XLMTokenizer, RobertaModel
+
+tokenizer = XLMTokenizer.from_pretrained("allegro/herbert-klej-cased-tokenizer-v1")
+model = RobertaModel.from_pretrained("allegro/herbert-klej-cased-v1")
+
+encoded_input = tokenizer.encode("Kto ma lepszą sztukę, ma lepszy rząd – to jasne.", return_tensors='pt')
+outputs = model(encoded_input)
+```
+
+## License
+CC BY-SA 4.0
+
+## Citation
+If you use this tokenizer, please cite the following paper:
+```
+@misc{rybak2020klej,
+    title={KLEJ: Comprehensive Benchmark for Polish Language Understanding},
+    author={Piotr Rybak and Robert Mroczkowski and Janusz Tracz and Ireneusz Gawlik},
+    year={2020},
+    eprint={2005.00630},
+    archivePrefix={arXiv},
+    primaryClass={cs.CL}
+}
+```
+Paper is accepted at ACL 2020, as soon as proceedings appear, we will update the BibTeX.
+
+## Authors
+Tokenizer was created by **Allegro Machine Learning Research** team.
+
+You can contact us at: <a href="mailto:[email protected]">[email protected]</a>