Add contribution guide

CONTRIBUTING.md

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+# Contributing
+
+Whether you’re a seasoned developer or new to Python, there are many ways you can contribute to PedPy. 
+If you’re interested in improving the library’s functionality, we encourage you to get involved in the development process. 
+Our code base is open source and available on GitHub, so you can easily contribute by submitting pull requests or reporting issues and bugs. 
+We also welcome feedback on new features or enhancements that you think would be useful.
+
+Fore more information check the Developer Guide on our website: [Developer Guide](https://pedpy.readthedocs.io/en/latest/developer_guide.html)