Improve and Clarify CONTRIBUTING.md for New Contributors

[ayushraistudio]

Our current CONTRIBUTING.md needs to be fixed and improved.

The goal is to make it clear, strict, and beginner-friendly so new contributors can easily understand how to contribute SVG themes without confusion.

What needs to be done:

• Rewrite and improve CONTRIBUTING.md
• Clearly explain how to add a new SVG theme
• Mention the exact file to edit: src/generateTrophy.js
• Clarify that contributors can work via GitHub web/mobile (local setup is optional)
• Add strict rules and constraints for SVG themes
• Explain how to test themes using the Vercel preview API
• List common mistakes that lead to PR rejection

Must include:

• SVG only (no images, no external fonts)
• Fixed size: 800×250
• Use only the provided data object
• No hardcoded usernames
• One theme per PR
• Correct SVG example with proper xmlns

Goal:

• Reduce confused contributors
• Reduce broken or low-quality PRs
• Increase beginner-friendly, clean contributions

File to modify:

• CONTRIBUTING.md