add HTTPS warning about time you'll need cert for

Created by: bbodenmiller

Add warning that user will need to maintain HTTPS cert. As can be seen at https://github.com/gitlabhq/gitlabhq/blob/master/app/controllers/application_controller.rb#L163-L167 Strict-Transport-Security is set to require a cert for the next 24 months of seeing the header from the GitLab application.