Add instructions for use with Cassandra 5.0 to documentation

[smarsching]

The newest version (0.9.14) of the agent seems to work fine with Cassandra 5.0, but there are a few additional steps that have to be taken when using it with this version of Cassandra:

1. The Netty Codec HTTP library (netty-codec-http-x.x.x.Final.jar) has to be copied to Cassandra’s lib directory.
2. When using Java 17, the following two lines have to be added to conf/jvm17-server.options:

--add-exports java.management/com.sun.jmx.interceptor=ALL-UNNAMED
--add-exports java.management/com.sun.jmx.mbeanserver=ALL-UNNAMED

I think it makes sense to add this information to the Usage / Agent section of the README.md file, so that Cassandra 5.0 users do not have to figure this out on their own.