Added description for dnserial.sh

Author: zer0rest

README.md

@@ -1,2 +1,5 @@
 # dns-monitor
 A series of bash/python scripts I wrote to monitor changes in my domains and alert myself.
+
+### dnserial.sh 
+This script receives the (sub)domain you want to monitor as an argument and monitors for changes to it's SOA record's serial number. If it finds such changes it publishes a message including the previous and current serial number to a specified MQTT topic for notifiers to consume.