Update README high-level consumer example

Update the example to show that upon exit, the consumer must call `close()`

Author: dfdeshom

README.md

@@ -35,10 +35,13 @@ from confluent_kafka import Consumer
 c = Consumer({'bootstrap.servers': 'mybroker', 'group.id': 'mygroup',
               'default.topic.config': {'auto.offset.reset': 'smallest'}})
 c.subscribe(['mytopic'])
-while True:
+running = True
+while running:
     msg = c.poll()
     if not msg.error():
         print('Received message: %s' % msg.value().decode('utf-8'))
+    else:
+        running = False
 c.close()
 ```