docs: add CONTRIBUTING badge to README.md

[PeterPedegru]

Add a CONTRIBUTING badge in README linking to CONTRIBUTING.md for easier discoverability of contribution guidelines.

What I did

Added a small CONTRIBUTING badge to the README header that links to the repository's CONTRIBUTING.md so that contribution instructions are immediately visible to visitors and potential contributors.

fixes: N/A

How I did it

• Inserted the following badge markdown into the top-of-readme badge block (next to existing CI/status badges):

[![Contributing](https://img.shields.io/badge/CONTRIBUTING-guidelines-brightgreen?style=flat-square)](https://github.com/ApeWorX/ape/blob/main/CONTRIBUTING.md)

• Committed the change on a new branch and opened this PR for review.
• Kept formatting consistent with existing badges (flat-square style and spacing).

How to verify it

1. Open the Pull Request preview or view the modified README.md file in the "Files changed" tab on GitHub.
2. Confirm the new CONTRIBUTING badge renders correctly in the README header.
3. Click the badge and verify it navigates to CONTRIBUTING.md in the main branch (or the file path used in the badge).
4. Verify spacing and visual alignment with adjacent badges (CI, license, etc.) to ensure consistent appearance.

Checklist

• All changes are completed
• Change is covered in tests (N/A — documentation-only change)
• Documentation is complete

[github-actions]

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