readme: * Install requirements
  ```
  pip install jieba
  ```
  
  * Generate words.txt
  ```bash
  data_dir=/path/to/wenetspeech
  # the data_dir contains:
  # tree -L 2 .
  # .
  # |-- TERMS_OF_ACCESS
  # |-- WenetSpeech.json
  # |-- audio
  #    |-- dev
  #    |-- test_meeting
  #    |-- test_net
  #    `-- train
  grep "\"text\":" $data_dir/WenetSpeech.json | sed -e 's/["text: ]*//g' > text.txt
  python -m jieba -d " " text.txt > tokenized.txt
  cat tokenized.txt | awk '{for(i=1;i<=NF;i++)print $i}' | sort | uniq > words.txt
  ```
  * Generate N-gram model
  ```
  ```