Add back the security section from v1

docs/content/contributing/submitting-security-issues.md

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+# Security
+
+## Security Advisories
+
+We strongly advise you to join our mailing list to be aware of the latest announcements from our security team.
+You can subscribe sending a mail to security+subscribe@traefik.io or on [the online viewer](https://groups.google.com/a/traefik.io/forum/#!forum/security).
+
+## CVE
+
+Reported vulnerabilities can be found on 
+[cve.mitre.org](https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=traefik).
+
+## Report a Vulnerability
+
+We want to keep Traefik safe for everyone.
+If you've discovered a security vulnerability in Traefik, we appreciate your help in disclosing it to us in a responsible manner, using [this form](https://security.traefik.io).