Add CONTRIBUTING.md for API development

[Chhunneng]

The apps/api/ folder does not have a contribution guide. Adding CONTRIBUTING.md will guide new developers on:

• How to set up a development environment
• Branching and PR conventions
• Running tests (pytest.ini, tests/)
• Writing and structuring new endpoints

[KanavCode]

Can I Create CONTRIBUTING.md FIle ??
I would like to give my Efforts in this issue

[AadityaBansal01]

Hi, I’d like to contribute to this issue. Could you please assign it to me and also label it with hacktoberfest?