Added CONTRIBUTING.md (#18)

* reverse string

* Formatted with psf/black

* remove whitespace

* Formatted with psf/black

* format code

* Formatted with psf/black

* add contribution

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

Author: realDuYuanChao

CONTRIBUTING.md

@@ -0,0 +1,27 @@
+# Contributing guidelines
+
+## Before contributing
+
+👏👏 Welcome to [examplehub/Python](https://github.com/examplehub/Python) ! Before sending your pull requests. Please make sure that you **read the whole guidelines**.
+
+## Contributing
+
+#### How to contribute
+1. Fork the project to your github account & clone locally computer.
+2. Create an upstream remote and sync your local copy before you branch.
+3. Branch for each separate piece of work.
+4. Do the work, write good commit messages, and read the `CONTRIBUTING.md`.
+5. Push to your origin repository.
+6. Create a new PR in GitHub.
+7. Waiting to be review by the maintainers.
+
+#### Code Style
+
+#### Code Style
+* Please follow [PEP 8](https://www.python.org/dev/peps/pep-0008/) Coding Style.
+* Single letter variable names are old school so please avoid them unless their life only spans a few lines.
+* Expand acronyms because `gcd()` is hard to understand but `greatest_common_divisor()` is not.
+* Write `test unit` to test all your functions.
+* [More details](https://docs.python.org/3/tutorial/controlflow.html#intermezzo-coding-style) about Code Style.
+
+### Thanks for your contribution 😊