add security vulnerability warning to 7.1

Add [security vulnerabily](https://about.gitlab.com/2014/08/28/gitlab-7-dot-2-1-security-release/) warning to 7.1 upgrade notes.
Small type on vulnerability. Also, I think it might be more clear if it read `GitLab 7.1 is affected by a XSS security vulnerability. See (blog post) for more details.`