git-codereview: document multiple-commit workflow

Also document a few recent changes that hadn't been added yet.

For golang/go#13651.

Change-Id: I79ec381abf34a385f42df40ee8450b413e0a3b9a
Reviewed-on: https://go-review.googlesource.com/19561
Reviewed-by: Austin Clements <austin@google.com>
1 file changed