udpate readme

Author: nick-zh

README.md

@@ -19,6 +19,10 @@ can help out to understand the internals of this library.
 - ext-rdkafka: >=4.0.0
 - librdkafka: >=0.11.6 (if you use `<librdkafka:1.x` please define your own error callback)
 
+:danger: Note: To use the transactional producer you'll need:
+- ext-rdkafka: >=4.1.0
+- librdkafka: >=1.4
+
 ## Installation
 ```
 composer require jobcloud/php-kafka-lib "~1.0"