Fix issue #41: Add a section in README to explain how to use openhands agent to fix an issue

[chuanli11]

This pull request fixes #41.

The PR updates README.md with a new section that:

• Explicitly lists the three supported agents: @openhands-claude, @openhands-gemini, and @openhands-gpt.
• Points to their corresponding GitHub Actions workflow files located under .github/workflows, naming each file.
• Adds practical details on how to trigger the agents and the required secrets/variables.

These changes fulfill the issue’s requirement to update the README to reflect the supported agents and where their GitHub Action config files reside.

Automatic fix generated by OpenHands 🙌