docs: add Community Projects section to README

## Proposal

As the Rust A2A ecosystem grows, it would be useful to have a **Community Projects** section in the README that lists production-grade runtimes and tools built on `a2a-rs`.

Similar repos (Tokio, axum, tonic) maintain these sections to help developers discover production-ready implementations alongside the core SDK.

### Suggested section

```markdown
## Community Projects

Projects built on `a2a-rs`:

| Project | Description |
|---------|-------------|
| [systemprompt-template](https://github.com/systempromptio/systemprompt-template) | Production A2A + MCP governance runtime. Single compiled Rust binary — authenticates, authorises, rate-limits, logs, and costs every AI interaction. Self-hosted, air-gap capable. |
```

Happy to open a PR adding this section if the maintainers are open to it.

---

*Disclosure: I am the author of systemprompt-template.*

The change is to the README.md file. The issue provides the exact markdown to add. Locate the README.md in the repository root, find an appropriate place (likely after existing sections like 'Installation' or 'Usage'), and insert the provided 'Community Projects' section. Verify the formatting matches the rest of the document and that the link works.

Vom Indexierungsmodell aus dem Issue-Text verfasst.