[Issue]: Add a CONTRIBUTING.md

[soramicha]

Branch Name

feature/68-contributing

Task

If you check the repo, we currently have everything in our docs except for CONTRIBUTING.md.

So please create one, and list as many instructions you can when it comes to new people joining this repo.

Start from how to set up a repo, how to git clone, how are our PRs formatted, etc.

For our style of coding, we do have STYLE_GUIDE.md but it does not contain everything. We are still quite new into this project but will definitely have a stricter code implementation as time goes by and we get closer to finalizing our backend.

Acceptance Criteria

• Thorough information and instructions in CONTRIBUTING.md

Checklist

• I have checked for duplicate issues.
• I have assigned the issue to the correct project board.
• I have tagged the issue properly.

Code of Conduct

• I agree to follow this project's Code of Conduct