docs: add Community Integrations section for Namecoin identity projects

[mstrofnone]

This is a small docs-only change that adds a "Community Integrations" section near the bottom of README.md (with a matching TOC entry), listing three third-party projects that wire Namecoin-based identity into strfry:

• strfry-namecoin-policy — write-policy plugin that verifies .bit NIP-05 identities via ElectrumX.
• strfry-nip05-namecoin — HTTP sidecar serving NIP-05 /.well-known/nostr.json from Namecoin records.
• namecoin-nostr-bridge — daemon that republishes on-chain Namecoin identity records as Nostr events.

The section explicitly notes these are third-party projects not maintained by the strfry authors, and invites further community integration PRs.

No code, build, or config changes — only README.md (15 lines added, 0 removed).

Note: the three linked repos are being published in parallel with this PR and may briefly 404; they should all be live within a few hours. Happy to adjust wording, move the section to docs/plugins.md, or drop it entirely if you'd prefer not to carry third-party links in the main README. Thanks for strfry!

[mstrofnone]

@hoytech if you would accept PRs for any of the above let me know.

[hoytech]

I just did some work to shrink the README down a bit, so I think I will make a new document in the docs/ directory, move this content there, and link to it from the README.

Thanks!