update version for composer (#28)

Author: nick-zh

README.md

@@ -50,7 +50,7 @@ compliant requests that can be used as well as high level abstractions to ease d
 This library is installed via [`composer`](http://getcomposer.org).
 
 ```bash
-composer require "flix-tech/confluent-schema-registry-api=^5.1"
+composer require "flix-tech/confluent-schema-registry-api=^6.0"
 ```
 
 > **NOTE**