Modify README for backend directory setupUpdate installation guide

[sheepyd]

Updated instructions for setting up environment variables.

[leonyangdev]

Updated instructions for setting up environment variables.

Hi @sheepyd，Thanks for the PR!

There’s just one small issue:

If you run cp env.example .env inside the backend directory and then execute:

docker-compose up -d

Compose won’t read backend/.env.
It only loads the .env file in the project root.

So the recommended approach is:

cp backend/.env.example .env

This ensures all environment variables are correctly loaded.

Feel free to update the PR—I'd be happy to review your next commit! 🚀