doc/install.html: how to update to a new release
doc/install.html: how to update to a new release

Fixes issue 422.

R=r
CC=golang-dev
http://codereview.appspot.com/180057

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