doc/contribute: add instructions to fix codereview on windows

Tell Windows users how to update Mercurial's library.zip to add some missing dependencies.

Fixes issue 4745.

R=golang-dev, patrick.allen.higgins, minux.ma, adg
CC=golang-dev
https://codereview.appspot.com/7558043

Committer: Andrew Gerrand <adg@golang.org>