Document unsigned packages before 2017-08-22
Document unsigned packages before 2017-08-22

Add to the signed packages documentation that any package prior to August 22, 2017 will NOT be signed, and an appropriate flag may be needed for the users package manager.

e.g. `yum --nogpgcheck`

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab