Add note to readme for difference between 1.x and 2.x releases (#12239)

Author: jaydeluca

README.md

@@ -67,6 +67,13 @@ This package includes the instrumentation agent as well as
 instrumentations for all supported libraries and all available data exporters.
 The package provides a completely automatic, out-of-the-box experience.
 
+*Note: There are 2.x releases and 1.x releases. The 2.0 release included significant breaking
+changes, the details of which can be found in the [release notes](https://github.com/open-telemetry/opentelemetry-java-instrumentation/releases/tag/v2.0.0).
+It is recommended to use the latest 2.x release which will have the latest features and improvements.
+1.x will receive security patches for a limited time and will not include other bug fixes and
+enhancements.*
+
+
 Enable the instrumentation agent using the `-javaagent` flag to the JVM.
 
 ```