apps/registration-agent: README_SNMP.txt: add more examples using MIB…

… and Nagios

apps/registration-agent/README_SNMP.txt

@@ -64,6 +64,22 @@ Here are some examples of snmp commands to query the agent:
   snmpwalk -c public -v 1  localhost .1.3.6.1.4.1.8072.9999
   snmpget -c public -v 1 -Onv localhost .1.3.6.1.4.1.8072.9999.1.1.2.0
 
+and if the MIB file is installed:
+
+  snmpget -c public -v 1 -Onv localhost \
+     REG-AGENT-MIB::reSIProcate.registrationAgent.registrationsFailed.0
+
+Nagios monitoring
+-----------------
+
+Here is an example Nagios command definition to query the SNMP sub-agent
+and raise a critical error if any registrations are in the failed state.
+
+define command {
+        command_name    check_snmp_sip_registrations_failed
+        command_line    /usr/lib/nagios/plugins/check_snmp -H '$HOSTADDRESS$' -o REG-AGENT-MIB::reSIProcate.registrationAgent.registrationsFailed.0 -c :0
+}
+
 Development process
 -------------------