Add documentation about random UUID for file uploads

Dev link: https://dev.gitlab.org/gitlab/gitlabhq/issues/2623

File uploads to any GitLab comment/description field receive a random UUID as a method of securing the file access. We need to document this fact in http://doc.gitlab.com/ce/security/README.html so users are aware of this. Based on our conversation in the referenced issue, we feel this is sufficiently secure method for file uploads.