Please document that words are not accounted for

[dancor]

Thank you for the work on this library. A big and easy improvement to its usefulness is to prominently document that it currently performs a character-level translation, which is incorrect for many common words and sentences.

>>> import pinyin
>>> pinyin.get('了解', format='numerical')
'le5jie3'

The output should be

'liao3jie3'