doc/contribute.html: mention hg mail during hg change discussion
doc/contribute.html: mention hg mail during hg change discussion

People keep not reading all the way to the bottom of the doc
and not running hg mail.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7137057

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