add security vulnerability warning to 7.1

Add [security vulnerability](https://about.gitlab.com/2014/08/28/gitlab-7-dot-2-1-security-release/) warning to 7.1 upgrade notes.
I tweaked the text slightly be still left it similar to the format of the other warnings: https://github.com/gitlabhq/gitlabhq/search?l=markdown&q=security+vulnerability+OR+vulnerabilities&type=Code&utf8=%E2%9C%93