Added Backend.md

[AnushkaKohli]

Fixes Issue

Closes #2006

Changes proposed

Added the BACKEND.md file describing how to contribute to backend.

Checklist

• [x] You've linked this PR to the correct issue.
• [x] You have checked that the code is working correctly.
• [x] You ⭐️ the repository!

Note to reviewers

[vercel[bot]]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
lets-animate-buttons	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 11, 2023 5:18am