umqtt.simple: Add standalone sub example.

umqtt.simple/example_sub.py

@@ -0,0 +1,30 @@
+import time
+from umqtt.simple import MQTTClient
+
+# Publish test messages e.g. with:
+# mosquitto_pub -t foo_topic -m hello
+
+# Received messages from subscriptions will be delivered to this callback
+def sub_cb(topic, msg):
+    print((topic, msg))
+
+def main(server="localhost"):
+    c = MQTTClient("umqtt_client", server)
+    c.set_callback(sub_cb)
+    c.connect()
+    c.subscribe(b"foo_topic")
+    while True:
+        if True:
+            # Blocking wait for message
+            c.wait_msg()
+        else:
+            # Non-blocking wait for message
+            c.check_msg()
+            # Then need to sleep to avoid 100% CPU usage (in a real
+            # app other useful actions would be performed instead)
+            time.sleep(1)
+
+    c.disconnect()
+
+if __name__ == "__main__":
+    main()