examples/README: Reference hole punching tutorial

[mxinden]

Description

Based on conversation at community call, this tutorial is hard to discover. Hope this will give it more visibility.

Links to any relevant issues

Open Questions

Change checklist

• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] A changelog entry has been made in the appropriate crates