docs: add CONTRIBUTING.md guidelines

[Dev1822]

Issue : #125

Description

This PR introduces a CONTRIBUTING.md file to the repository to help new contributors get started with the project quickly and maintain code quality standards.

Changes Made

• Added step-by-step setup instructions for both the Expo frontend and Node.js backend.
• Provided guidelines for setting up environment variables.
• Established branch naming conventions.
• Standardized commit message formatting (using Conventional Commits).
• Documented the process for submitting a Pull Request.
• Outlined basic code style standards.

Motivation and Context

Previously, the repository lacked clear guidelines for new contributors. This addition will make it significantly easier for developers to fork, run the project locally, and submit properly formatted Pull Requests, ultimately streamlining the review process.