Add Contributing file

CONTRIBUTING.md

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+# Contributing to Opfunu
+
+We appreciate your interest in contributing to Opfunu! This guide details how to contribute in a way that is efficient 
+for everyone.
+
+## Table of Contents
+
+- [Code of Conduct](#code-of-conduct)
+- [Getting Started](#getting-started)
+- [How Can I Contribute?](#how-can-i-contribute)
+- [Pull Requests](#pull-requests)
+- [Issue Tracking](#issue-tracking)
+
+## Code of Conduct
+
+All contributors are expected to adhere to the project's [Code of Conduct](CODE_OF_CONDUCT.md). Please read the document before contributing. 
+
+## Getting Started
+
+1. Fork the project repository and clone your fork:
+
+    ```
+    git clone https://github.com/thieu1995/opfunu.git
+    ```
+
+2. Create a new branch for your changes:
+
+    ```
+    git checkout -b name-of-your-branch
+    ```
+
+3. Make your changes and commit them:
+
+    ```
+    git commit -m "Detailed commit message"
+    ```
+
+4. Push your changes to your fork:
+
+    ```
+    git push origin name-of-your-branch
+    ```
+
+5. Create a pull request from your branch to the Opfunu main branch.
+
+## How Can I Contribute?
+
+Here are some ways to contribute:
+
+- Improve documentation
+- Fix bugs or add new features
+- Write tutorials or blog posts
+- Review code submissions
+- Test the application and report issues
+
+However, before contributing, make sure that the unit tests pass and that new functionality is covered by unit tests. 
+The unit tests can be run using `pytest`. Change working directory to opfunu and then use:
+
+```python
+# Test CEC-based functions
+python -m pytest tests/cec_based
+
+# Test Name-based functions
+python -m pytest tests/name_based
+```
+
+Or you can test all files by:
+
+```python
+python -m pytest
+```
+
+
+## Pull Requests
+
+[Pull requests](https://github.com/thieu1995/opfunu/pulls) are the best way to propose changes to the codebase. We 
+actively welcome your pull requests:
+
+1. Fork the repo and create your branch from `main`.
+2. If you've added code that should be tested, add tests.
+3. If you've changed APIs, update the documentation.
+4. Ensure the test suite passes.
+5. Issue that pull request!
+
+## Issue Tracking
+
+We use [GitHub issues](https://github.com/thieu1995/opfunu/issues) to track public bugs and requests. Please ensure 
+your description is clear and has sufficient instructions to be able to reproduce the issue.
+
+## Any questions?
+
+Don't hesitate to contact us if you have any questions. Contact [@thieu1995](mailto:[email protected])
+or ask your question on issues.
+
+Thank you for your contributions!