doc/contribute: reorder documented steps to match normal workflow.
doc/contribute: reorder documented steps to match normal workflow.

LGTM=adg
R=golang-codereviews, bradfitz, adg
CC=golang-codereviews
https://codereview.appspot.com/109970043

Committer: Shenghou Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>

wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang