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/jannesg/bertsson/README.md

README.md

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+---
+language: sv
+---
+
+# BERTSSON Models
+
+The models are trained on:
+- Government Text
+- Swedish Literature
+- Swedish News
+
+Corpus size: Roughly 6B tokens.
+
+The following models are currently available:
+
+- **bertsson** - A BERT base model trained with the same hyperparameters as first published by Google.
+
+All models are cased and trained with whole word masking.
+
+Stay tuned for evaluations.