Add a README for the h3 example

[dahlia]

examples/h3 contains a working example, but it has no README. That makes it harder for new contributors and users to understand what the example is meant to show.

Add examples/h3/README.md with a short description, setup steps, run command, and the actor URL that can be looked up.

Suggested check: hongdown --check examples/h3/README.md.

[dkxmercury]

Hi! I'd like to work on this issue - adding a README.md for the h3 example. Is it open for outside contributors, or is it reserved for the OSSCA 2026 program? If it's available, I'll pick it up. Thanks!

[dahlia]

Sorry, it's reserved for OSSCA 2026 program!

[dkxmercury]

No problem at all, thanks for the quick reply and for running the OSSCA program! I'll pick up one of the other open issues instead. Thanks for fedify, it's a great project.

[m2nsp]

Hi, I'd like to work on this issue. Could you assign it to me?

[dahlia]

@m2nsp Assigned, thanks!