Add readme with security policy link

Author: per1234

README.md

@@ -0,0 +1,11 @@
+# mdns-discovery
+
+MDNS (Bonjour) pluggable discovery tool
+
+## Security
+
+If you think you found a vulnerability or other security-related bug in this project, please read our
+[security policy](https://github.com/arduino/mdns-discovery/security/policy) and report the bug to our Security Team 🛡️
+Thank you!
+
+e-mail contact: security@arduino.cc