Update README.md to add idlelatency.sh script

Author: richb-hanover

README.md

@@ -14,6 +14,8 @@ and then restore the full set of packages after the upgrade.
 
 * [betterspeedtest.sh](#betterspeedtestsh) & [netperfrunner.sh](#netperfrunnersh) & [networkhammer.sh](#networkhammersh) - scripts that measure the performance of your router or offer load to the network for testing.
 
+* [idlelatency.sh](#idlelatencysh) - a script to measure the latency of an "idle line" without any additional traffic generation from the script.
+
 * [tunnelbroker.sh](#tunnelbrokersh) - a script to set up a IPv6 6-in-4 tunnel to TunnelBroker.net. 
 
 These scripts can be saved in the `/usr/lib/OpenWrtScripts` directory. 
@@ -162,6 +164,34 @@ On the right is a test using SQM: the latency goes up a little (less than 23 mse
         90pct: 5163.901                                              90pct: 56.061
           Max: 5334.262                                                Max: 69.333
 
+---
+## [idlelatency.sh](https://github.com/richb-hanover/OpenWrtScripts/blob/master/idlelatency.sh)
+
+The `idlelatency.sh` script summarizes ping times measured over a specified time interval. To invoke the script:
+
+    sh idlelatency.sh [ -4 | -6 ] [ -t duration ] [ -p host-to-ping ] 
+    
+Options, if present are:
+
+* -4 | -6: Enable ipv4 or ipv6 testing (default - ipv4)
+* -t | --time: Duration for how long each direction's test should run - (default - 60 seconds)
+* -p | --ping: Host to ping to measure latency (default - gstatic.com)
+
+The output of the script looks like this:
+
+```
+root@openwrt: sh idlelatency.sh
+2020-05-02 12:10:53 Testing idle line while pinging gstatic.com (60 seconds)
+............................................................
+  Latency: (in msec, 60 pings, 0.00% packet loss)
+      Min: 20.438
+    10pct: 22.633
+   Median: 36.907
+      Avg: 35.143
+    90pct: 45.994
+      Max: 50.377
+```
+
 ---         
 ## [netperfrunner.sh](https://github.com/richb-hanover/OpenWrtScripts/blob/master/netperfrunner.sh)