add redpanda badge (#42)

Author: nick-zh

README.md

@@ -1,7 +1,8 @@
 # PHP Kafka extension (php-simple-kafka-client)
 
 [![Supported librdkafka versions: >= 1.4.0](https://img.shields.io/badge/librdkafka-%3E%3D%201.4.0-blue.svg)](https://github.com/edenhill/librdkafka/releases) 
-[![Supported Kafka versions: >= 0.9](https://img.shields.io/badge/kafka-%3E%3D%200.9-blue.svg)](https://github.com/edenhill/librdkafka/blob/master/INTRODUCTION.md#broker-version-compatibility) 
+[![Supported Kafka versions: >= 0.9](https://img.shields.io/badge/kafka-%3E%3D%200.9-blue.svg)](https://github.com/edenhill/librdkafka/blob/master/INTRODUCTION.md#broker-version-compatibility)
+![Supported Redpanda versions: >= 20.x](https://img.shields.io/badge/redpanda-%3E%3D20.x-red) 
 ![Supported PHP versions: 7.x .. 8.x](https://img.shields.io/badge/php-7.x%20..%208.x-blue.svg)
 [![License: BSD-3](https://img.shields.io/badge/License-BSD--3-green.svg)](https://github.com/php-kafka/php-simple-kafka-client/blob/main/LICENSE) 
 [![Join the chat at https://gitter.im/php-kafka/php-simple-kafka-client](https://badges.gitter.im/php-kafka/php-simple-kafka-client.svg)](https://gitter.im/php-kafka/php-simple-kafka-client?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)