Add example for Jitsi JVB STUN server

roles/matrix-jitsi/defaults/main.yml

@@ -238,6 +238,11 @@ matrix_jitsi_jvb_rtp_tcp_port: 4443
 # matrix_jitsi_jvb_custom_config_extension: |
 #   org.jitsi.videobridge.xmpp.user.shard.DISABLE_CERTIFICATE_VERIFICATION=false
 #   org.jitsi.videobridge.ENABLE_STATISTICS=false
+# One possible configuration is to add a STUN for the Jitsi JVB to use in case you are behind a NAT, 
+# as mentioned here: https://jitsi.github.io/handbook/docs/devops-guide/devops-guide-quickstart/#advanced-configuration
+# For example you can use a Google STUN:
+# matrix_jitsi_jvb_custom_config_extension: |
+#   org.ice4j.ice.harvest.STUN_MAPPING_HARVESTER_ADDRESSES=stun.l.google.com:19302
 matrix_jitsi_jvb_custom_config_extension: ''
 
 # Additional environment variables to pass to the Jitsi JVB container.