Update Readme instructions for using shaded variants.

Consider using https://github.com/ryandens/javaagent-gradle-plugin#application-plugin-integration to show how to use the shaded variant as an agent (i.e. what we recommend) rather than adding it to the application classpath.

You can write up similar instructions for Maven by adapting https://stackoverflow.com/questions/14777909/specify-javaagent-argument-with-maven-exec-plugin, but it's a lot more verbose. It's up to you whether you want to do that.

_Originally posted by @punya in https://github.com/GoogleCloudPlatform/opentelemetry-operations-java/pull/245#discussion_r1192031907_