      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
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go