doc/contribute: try to make it clearer that you send a review with hg mail, not hg submit.
doc/contribute: try to make it clearer that you send a review with hg mail, not hg submit.

R=r
http://codereview.appspot.com/171046

apparentlymart/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,tianon/debian-golang,tv42/old-go,nightlyone/golang,tianon/debian-golang,tianon/debian-golang,dsriram/golang,bdwalton/debian-golang,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,h8liu/golang,tianon/debian-golang,dsriram/golang,bdwalton/debian-golang,dsriram/golang,gnanderson/go,ssrl/go,h8liu/golang,bdwalton/debian-golang,h8liu/golang,pombredanne/go-deleteme,bdwalton/debian-golang,tianon/debian-golang,nightlyone/golang,tianon/debian-golang,nightlyone/golang,ssrl/go,nightlyone/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,h8liu/golang,apparentlymart/golang,tv42/old-go,h8liu/golang,gnanderson/go,bdwalton/debian-golang,bdwalton/debian-golang,gnanderson/go,pombredanne/go-deleteme,ssrl/go,tianon/debian-golang,h8liu/golang,nightlyone/golang,tv42/old-go,bdwalton/debian-golang,dsriram/golang,nightlyone/golang,apparentlymart/golang,ssrl/go,tv42/old-go,gnanderson/go,dsriram/golang,dsriram/golang,gnanderson/go,h8liu/golang,pombredanne/go-deleteme,tianon/debian-golang,ssrl/go,pombredanne/go-deleteme,tv42/old-go,nightlyone/golang,pombredanne/go-deleteme,bdwalton/debian-golang,dsriram/golang,gnanderson/go,ssrl/go