doc/contribute: mention 'hg help codereview'
doc/contribute: mention 'hg help codereview'

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/6932053

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang