Add generic CLA links to contributing.md
As per https://gitlab.com/gitlab-com/finance/issues/67, this MR adds the generic CLA links to the contributing.md file
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
As per https://gitlab.com/gitlab-com/finance/issues/67, this MR adds the generic CLA links to the contributing.md file