Add MCP Toplist rank badge

[chrstphe]

Hi! MCP Toplist tracks 81,919 MCP servers across the major
registries, and OpenSumi MCP Server is currently ranked #837.

This PR adds a small live badge to your README showing that rank — it updates
automatically as the leaderboard changes:

Totally fine to close if you'd rather not — and if you'd like your server's
data corrected or removed from the leaderboard, just say so here or open an
issue at the site. Thanks for building OpenSumi MCP Server!

Summary by CodeRabbit

• 文档
  • 在 README 顶部新增 MCP Toplist 徽标，并保留适当间距。

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: c0dc0ae7-3e3f-4b7c-ab4c-9e7b3a960ec8

📥 Commits

Reviewing files that changed from the base of the PR and between 9fee6aa and e503d3d.

📒 Files selected for processing (1)

• README.md

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Walkthrough

README.md 顶部新增 MCP Toplist 徽标链接，并在徽标后保留空行。

Changes

MCP Toplist 徽标

Layer / File(s)	Summary
README 徽标入口 README.md	在 README.md 顶部新增 MCP Toplist 徽标链接及空行。

Estimated code review effort: 1 (Trivial) | ~2 minutes

Possibly related PRs

• opensumi/core#4497：同样更新 README 中的 MCP 相关徽标。

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	标题准确概括了本次变更：在 README 中新增 MCP Toplist 排名徽标。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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Warning

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