      doc: mention 'hg update default' in contribution guidelines
      doc: mention 'hg update default' in contribution guidelines

Fixes issue 3634.

R=golang-dev, bsiegert, rsc
CC=golang-dev
http://codereview.appspot.com/6270047

apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,tv42/old-go,gnanderson/go,dsriram/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,tv42/old-go,tv42/old-go,gnanderson/go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme