docs: reference ClawBench as an external benchmark

[reacher-z]

Summary

Adds a clearly labeled external benchmark reference beside AndroidWorld's existing MiniWoB++ benchmark note.

The ClawBench entry links its paper, code, project page, and task-definition datasets, and uses neutral wording. It explicitly states that ClawBench is not integrated with or supported by AndroidWorld; no AndroidWorld task, evaluator, environment, or support claim is added.

Verification

• Documentation-only change to README.md
• git diff --check passes
• Submitted by reacher-z, a ClawBench maintainer; text prepared with OpenAI Codex assistance
• Maintainer edits are enabled on the fork

No issue is closed by this PR.

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