Add Boot Compatibility Caveat

Author: garyrussell

src/reference/asciidoc/appendix.adoc

@@ -5,6 +5,8 @@ When using Spring for Apache Kafka in a Spring Boot application, the Kafka depen
 If you wish to use a different version, you need to override all of the associated dependencies.
 This is especially true when using the embedded Kafka broker in `spring-kafka-test`.
 
+IMPORTANT: Backwards compatibility is not supported for all Boot versions; Spring for Apache Kafka 2.6 has been tested with Spring Boot 2.4 and 2.3.
+
 **maven**
 
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