docs: add self-hosted deployment guide link in README

## Suggestion

It would be helpful to add a link to the self-hosted deployment documentation in the README, so users who want to run their own instance can quickly find the setup guide.

Something like:

```markdown
## Self-Hosted Deployment

For users who prefer to self-host, see the [Self-Hosted Guide](https://qwenpaw.agentscope.io/self-hosted).
```

This would make the onboarding experience smoother for new users exploring the project.