note on testing the kafka bridge

Author: evictor

README.md

@@ -280,4 +280,11 @@ $pubsub = $manager->connection('custom_connection_name');
 
 Run `composer install` to ensure local installation of dependencies.
 
-Run `make tests` to run test suite.
+Run `make tests` to run test suite.
+
+If you need to run Kafka tests, install the Kafka bridge (note you still need to install the C library and PHP extension
+separately):
+
+```bash
+composer require superbalist/php-pubsub-kafka
+```