docs: add alpha callout to README

[Alek99]

Replaces the plain "XY is early alpha, and is receiving frequent enhancements. Any contributions are appreciated!" sentence in README.md with a GitHub [!IMPORTANT] callout:

XY is in alpha and is receiving frequent enhancements.
⭐️ Star the repo to follow the progress.

README-only change; no code or behavior affected.

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Generated by Claude Code

Summary by CodeRabbit

• Documentation
  • Updated the README to clarify that XY is in alpha.
  • Added a prompt for readers to star the repository to follow progress.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 8411ee2e-dc08-4d4d-92f2-e0415609287c

📥 Commits

Reviewing files that changed from the base of the PR and between 45deedb and 446b4c8.

📒 Files selected for processing (1)

• README.md

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📝 Walkthrough

Walkthrough

The README introduction now presents the project’s alpha status in an IMPORTANT block and asks readers to star the repository for progress updates.

Changes

README Messaging

Layer / File(s)	Summary
Alpha status notice README.md	Replaces the plain-text alpha notice with formatted messaging that states “XY is in alpha” and prompts readers to star the repository.

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

Possibly related PRs

• reflex-dev/xy#346: Updates the README introduction with related alpha-status and introductory copy changes.

Suggested reviewers: palashawasthi05, farhanaliraza

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/xy-alpha-callout-readme-m9m5dx

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_{Comment @coderabbitai help to get the list of available commands.}

[codspeed-hq]

Merging this PR will not alter performance

✅ 103 untouched benchmarks
⏩ 2 skipped benchmarks¹

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_{Comparing claude/xy-alpha-callout-readme-m9m5dx (446b4c8) with main (45deedb)}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩