Add security policy to repo (#12536)

SECURITY.md

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+# Reporting security issues
+
+The Gitea maintainers take security seriously.  
+If you discover a security issue, please bring it to their attention right away!
+
+### Reporting a Vulnerability
+
+Please **DO NOT** file a public issue, instead send your report privately to `security@gitea.io`.
+
+Security reports are greatly appreciated and we will publicly thank you for it, although we keep your name confidential if you request it.