Add Environment Variables Documentation to README

[shrushtigaynar]

The README currently does not document the environment variables required to run the backend application.

Contributors may encounter setup issues because variables such as MongoDB connection strings, session secrets, or API keys are not explained.

Suggested improvements:

• Add an Environment Variables section to the README.
• Create a .env.example file containing required variables.
• Briefly explain the purpose of each variable.

This will improve onboarding and make local development setup easier for contributors.

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