go-tour: add TODO list of new content to be added
go-tour: add TODO list of new content to be added

Somewhat based on this list:
https://code.google.com/p/go-tour/issues/detail?id=21

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/10860043

BenjaminTsai/go-tour-zh-tw,BenjaminTsai/go-tour-zh-tw