Update CONTRIBUTING.md to refer to the review checklist
Update CONTRIBUTING.md to refer to the review checklist

Signed-off-by: Tim Waugh <fe049355006bf7fdd11c5e47ffe7a5032af30704@redhat.com>

projectatomic/osbs-client,DBuildService/osbs-client,projectatomic/osbs-client,DBuildService/osbs-client