docs: add CONTRIBUTING.md and .env.example for better onboarding

.env.example

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+# -------------------------------------------------------
+# Eye Lab: Gaze Tracker API — Environment Variables
+# -------------------------------------------------------
+# Copy this file to .env and fill in your actual values:
+#   cp .env.example .env
+#
+# ⚠️  Never commit your .env file — it's in .gitignore
+# -------------------------------------------------------
+
+
+# --- Flask ---
+# Set to 'development' for local dev, 'production' for deployment
+FLASK_ENV=development
+
+# Set to 'true' to enable Flask debug mode (development only)
+FLASK_DEBUG=true
+
+# Port Flask will run on (default: 5000)
+FLASK_RUN_PORT=5000
+
+
+# --- Firebase ---
+# Your Firebase project credentials
+# Get these from: Firebase Console → Project Settings → General
+FIREBASE_API_KEY=your-firebase-api-key
+FIREBASE_AUTH_DOMAIN=your-project-id.firebaseapp.com
+FIREBASE_PROJECT_ID=your-project-id
+FIREBASE_STORAGE_BUCKET=your-project-id.appspot.com
+FIREBASE_MESSAGING_SENDER_ID=your-messaging-sender-id
+FIREBASE_APP_ID=your-firebase-app-id
+
+# Firebase Cloud Function base URL (used for API calls to Firebase Functions)
+# Example: https://us-central1-your-project-id.cloudfunctions.net
+FIREBASE_FUNCTION_URL=https://us-central1-your-project-id.cloudfunctions.net
+
+
+# --- Google Cloud (if deploying to GCP) ---
+# Your Google Cloud project ID
+GOOGLE_CLOUD_PROJECT=your-gcp-project-id
+
+
+# --- Model / ML Config ---
+# Path to store trained gaze estimation models
+MODEL_STORAGE_PATH=./models
+
+# Temp directory for batch prediction files
+TEMP_DIR=./tmp

CONTRIBUTING.md

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+# Contributing to Eye Lab: Gaze Tracker API
+
+Thank you for your interest in contributing! 🎉  
+Eye Lab is an open-source eye-tracking backend built with Python and Flask. Every contribution — big or small — is welcome.
+
+---
+
+## 📋 Table of Contents
+
+- [Getting Started](#getting-started)
+- [Setting Up Locally](#setting-up-locally)
+- [How to Contribute](#how-to-contribute)
+- [Branch Naming](#branch-naming)
+- [Commit Message Style](#commit-message-style)
+- [Pull Request Guidelines](#pull-request-guidelines)
+- [Issue Guidelines](#issue-guidelines)
+- [Code Style](#code-style)
+
+---
+
+## Getting Started
+
+1. **Fork** this repository on GitHub
+2. **Clone** your fork locally:
+   ```bash
+   git clone https://github.com/<your-username>/eye-tracker-api.git
+   cd eye-tracker-api
+   ```
+3. **Add upstream remote** to stay in sync:
+   ```bash
+   git remote add upstream https://github.com/ruxailab/eye-tracker-api.git
+   ```
+
+---
+
+## Setting Up Locally
+
+### Prerequisites
+- Python 3.x
+- pip
+
+### Steps
+
+```bash
+# Create and activate a virtual environment
+python3 -m venv venv
+source venv/bin/activate        # Linux/macOS
+venv\Scripts\activate           # Windows
+
+# Install dependencies
+pip install -r requirements.txt
+
+# Copy environment variables
+cp .env.example .env
+# Edit .env and fill in your values
+
+# Run the Flask app
+flask run
+```
+
+> **Note:** The project has a frontend counterpart at [web-eye-tracker-front](https://github.com/ruxailab/web-eye-tracker-front). Install it as well for the full application experience.
+
+---
+
+## How to Contribute
+
+### Working on an existing issue
+1. Go to the [Issues](https://github.com/ruxailab/eye-tracker-api/issues) tab
+2. Find an unassigned issue you want to work on
+3. **Comment on it** expressing your interest — the first person to comment gets preference
+4. Wait for a maintainer to assign it to you before starting work
+
+### Raising a new issue
+1. Check that the issue doesn't already exist
+2. Open a new issue with a clear title and description
+3. The person who raises an issue has **preference** to fix it
+4. Tag it appropriately (`bug`, `enhancement`, `documentation`, etc.)
+
+---
+
+## Branch Naming
+
+Use the following convention:
+
+| Type | Format | Example |
+|------|--------|---------|
+| Bug fix | `fix/<short-description>` | `fix/hardcoded-file-paths` |
+| New feature | `feat/<short-description>` | `feat/auto-model-selection` |
+| Documentation | `docs/<short-description>` | `docs/add-contributing-guide` |
+| Refactor | `refactor/<short-description>` | `refactor/rename-typo-function` |
+| Tests | `test/<short-description>` | `test/add-predict-unit-tests` |
+
+---
+
+## Commit Message Style
+
+Follow [Conventional Commits](https://www.conventionalcommits.org/):
+
+```
+<type>: <short description>
+
+# Examples:
+fix: remove hardcoded Firebase URL and use environment variable
+feat: add auto model selection for gaze estimation
+docs: add CONTRIBUTING.md and .env.example
+refactor: rename trian_and_predict to train_and_predict
+test: add smoke test for Flask app startup
+```
+
+**Types:** `fix`, `feat`, `docs`, `refactor`, `test`, `chore`, `perf`
+
+---
+
+## Pull Request Guidelines
+
+1. **One PR per issue** — keep your changes focused
+2. **Link the issue** in your PR description using `Closes #<issue-number>`
+3. **Describe your changes** clearly — what you changed and why
+4. **Test your changes** locally before submitting
+5. **Keep PRs small** — large PRs are harder to review and less likely to be merged quickly
+
+### PR Description Template
+
+```markdown
+## What does this PR do?
+<Brief description of the change>
+
+## Related Issue
+Closes #<issue-number>
+
+## Changes Made
+- <change 1>
+- <change 2>
+
+## How to Test
+<Steps to verify the change works>
+```
+
+---
+
+## Issue Guidelines
+
+When opening an issue, please include:
+
+- **Clear title** summarizing the problem or proposal
+- **Description** of what's happening vs. what's expected
+- **Steps to reproduce** (for bugs)
+- **Proposed solution** (if you have one)
+- **Your environment** (OS, Python version) for bugs
+
+---
+
+## Code Style
+
+- Follow [PEP 8](https://peps.python.org/pep-0008/) for Python code
+- Use meaningful variable and function names
+- Add docstrings to new functions
+- Remove unused imports before submitting
+
+---
+
+## Need Help?
+
+- Join our [Discord](https://discord.gg/fDFwcE6WD2) and introduce yourself
+- Ask questions in [GitHub Discussions](https://github.com/ruxailab/RUXAILAB/discussions)
+- Email us at `ruxailab@gmail.com`
+
+We look forward to your contributions! 🚀