add package defaults link to README.md
Minor improvement to better expose package defaults
Related to https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1070 & https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1870
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Minor improvement to better expose package defaults
Related to https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1070 & https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1870