mosquitto_sub: add page (#2718)

pages/common/mosquitto_sub.md

@@ -0,0 +1,15 @@
+# mosquitto_sub
+
+> A simple MQTT version 3.1.1 client that will subscribe to topics and print the messages that it receives.
+
+- Subscribe to the topic `sensors/temperature` information with Quality of Service (`QoS`) set to 1. (The default hostname is `localhost` and port `1883`):
+
+`mosquitto_sub -t {{sensors/temperature}} -q {{1}}`
+
+- Subscribe to all broker status messages publishing on `iot.eclipse.org` port `1885` and print published messages verbosely:
+
+`mosquitto_sub -v -h "iot.eclipse.org" -p 1885 -t {{\$SYS/#}}`
+
+- Subscribe to multiple topics matching a given pattern. (+ takes any metric name):
+
+`mosquitto_sub -t {{sensors/machines/+/temperature/+}}`