Added flush() call to AvroProducer example (closes confluentinc#221)

Author: edenhill

README.md

@@ -78,6 +78,7 @@ key = {"name": "Key"}
 
 avroProducer = AvroProducer({'bootstrap.servers': 'mybroker,mybroker2', 'schema.registry.url': 'http://schem_registry_host:port'}, default_key_schema=key_schema, default_value_schema=value_schema)
 avroProducer.produce(topic='my_topic', value=value, key=key)
+avroProducer.flush()
 ```
 
 **AvroConsumer**