build: add codereview.cfg
build: add codereview.cfg

Change-Id: Ib58b78b005768bb56ef2022d297ff52c0f0625f8
Reviewed-on: https://upspin-review.googlesource.com/15760
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/b2