readme: ## How to build a dummy model
  
  
  ```python
  from transformers BertConfig, BertForMaskedLM, BertTokenizer, TFBertForMaskedLM
  
  SMALL_MODEL_IDENTIFIER = "julien-c/bert-xsmall-dummy"
  DIRNAME = "./bert-xsmall-dummy"
  
  config = BertConfig(10, 20, 1, 1, 40)
  
  model = BertForMaskedLM(config)
  model.save_pretrained(DIRNAME)
  
  tf_model = TFBertForMaskedLM.from_pretrained(DIRNAME, from_pt=True)
  tf_model.save_pretrained(DIRNAME)
  
  # Slightly different for tokenizer.
  # tokenizer = BertTokenizer.from_pretrained(DIRNAME)
  # tokenizer.save_pretrained()
  ```
  