docs(CONTRIBUTING.md): how to build document site (#2355)
docs(CONTRIBUTING.md): how to build document site (#2355)


alanhamlett/c3,alanhamlett/c3,c3js/c3,c3js/c3,c3js/c3,c3js/c3,masayuki0812/c3,masayuki0812/c3,alanhamlett/c3