Merge branch 'securitytxt' into 'master'

Add a security txt https://securitytxt.org/

See merge request gitlab-org/gitlab-ce!27961

security.txt

+Contact: security@gitlab.com
+Acknowledgments: https://about.gitlab.com/security/vulnerability-acknowledgements/
+Preferred-Languages: en
+Canonical: https://about.gitlab.com/security/disclosure/
+Policy: https://hackerone.com/gitlab
+Hiring: https://about.gitlab.com/jobs/