Add SECURITY.md

SECURITY.md

+# Security
+
+## Reporting security issues
+
+We take security very seriously and develop all Jitsi projects to be secure and safe.
+
+If you find (or simply suspect) a security issue in any of the Jitsi projects, please send us an email to security@jitsi.org.
+
+**We encourage responsible disclosure for the sake of our users, so please reach out before posting in a public space.**