docs: add Contributing section to README.md

[AdekunleBamz]

Summary

Adds a dedicated "Contributing" section to the README.md that links to the detailed CONTRIBUTING.md guide and provides community links.

Problem

The README.md lacked a prominent "Contributing" section, making it harder for potential contributors to find contribution guidelines.

Solution

Added a clear Contributing section before the Copyright section, following the pattern of other open-source projects.

Testing

• Verified the link to CONTRIBUTING.md works
• Confirmed Discord link matches existing badge
• Checked formatting consistency with rest of README

[CLAassistant]

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