Improve README instructions

[beckermarc]

This PR improves the instructions found in the README on how to integrate the library into a Maven-based project:

• Dependency can be declared as optional, to make it available at compilation time, but to prevent it from being present at runtime.
• Mention annotation processor prerequisite. The annotation processor path needs to be explicitly configured, in case other annotation processors paths are already explicitly configured in the Maven compiler plugin. Explicitly configuring annotation processors is also required as of JDK 23.

Testing done

Figured this out, while trying to use this plugin in my own project.
After doing these additional steps, things started to work! 🥳

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed