Add preliminary SECURITY.md

SECURITY.md

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+If you think you have found a security problem with Nethogs,
+please report it privately to arnout@bzzt.net .
+
+Nethogs is meant to be used by trusted users: it requires
+root (or various capabilities) to function. We don't
+encourage making nethogs suid root and recommend explicitly
+giving the users that should be able to run nethogs the
+necessary permissions instead. We don't consider privilege
+escalation via a suid nethogs a vulnerability, though we
+might still fix them as security hardening improvements when
+reported.
+
+Running nethogs on a machine processing untrusted network
+traffic or allowing untrusted users is supported to a point:
+it is expected that attackers with such authorizations can
+cause resource usage / denial of service conditions in nethogs,
+but it should not allow attackers to gain privilege escalation
+though this path.
+
+As for correctness, nethogs is meant as a "quick overview" utility,
+not as a reliable accounting tool. It is possible that an untrusted
+user could shape their traffic/network usage patterns to
+circumvent reliable recording by nethogs.