umqtt.simple: Add standalone pub example.

umqtt.simple/example_pub.py

@@ -0,0 +1,13 @@
+from umqtt.simple import MQTTClient
+
+# Test reception e.g. with:
+# mosquitto_sub -t foo_topic
+
+def main(server="localhost"):
+    c = MQTTClient("umqtt_client", server)
+    c.connect()
+    c.publish(b"foo_topic", b"hello")
+    c.disconnect()
+
+if __name__ == "__main__":
+    main()