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_

Start with the repository README and review the application-plugin integration example linked in the issue. Update the instructions to recommend using the shaded variant as a Java agent rather than placing it on the application classpath; optionally add Maven guidance based on the linked Maven exec-plugin example. Done means the README clearly documents the recommended usage.

Written by the indexing model from the issue text.