Adds add_callback/add_errback example to docs (#1441)

Author: Berkodev

docs/usage.rst

@@ -91,6 +91,18 @@ KafkaProducer
     for _ in range(100):
         producer.send('my-topic', b'msg')
 
+    def on_send_success(record_metadata):
+        print(record_metadata.topic)
+        print(record_metadata.partition)
+        print(record_metadata.offset)
+
+    def on_send_error(excp):
+        log.error('I am an errback', exc_info=excp)
+        # handle exception
+
+    # produce asynchronously with callbacks 
+    producer.send('my-topic', b'raw_bytes').add_callback(on_send_success).add_errback(on_send_error)
+
     # block until all async messages are sent
     producer.flush()