doc: move single change workflow note in contribution guide

The note about the single change workflow is included in the
git-codereview installation instructions, but it has nothing to do with
installing git-codereview. This note is more relevant for when a change
is actually being made.

Change-Id: Iccb90f3b7da87fab863fa4808438cd69a21a2fce
Reviewed-on: https://go-review.googlesource.com/76317


Reviewed-by: Steve Francia <spf@golang.org>