README improvements
README improvements

* Remove duplicate Docker section
* Mention pulling the latest built image

outoftime/learnpad,jwang1919/popcode,outoftime/learnpad,popcodeorg/popcode,popcodeorg/popcode,jwang1919/popcode,jwang1919/popcode,popcodeorg/popcode,popcodeorg/popcode,jwang1919/popcode