Add contributing documentation

Author: TrafeX

CONTRIBUTING.md

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+# Contributing to this project
+I love your input! I want to make contributing to this project as easy and transparent as possible.
+
+## Report bugs using Github's [issues](https://github.com/TrafeX/docker-php-nginx/issues)
+GitHub issues is used to track public bugs.
+Report a bug by [opening a new issue](https://github.com/TrafeX/docker-php-nginx/issues/new); it's that easy!
+
+### What to include in an issue?
+- A quick summary and/or background
+- Steps to reproduce
+    - Include what version of the image you are using
+    - Add the `Dockerfile` if you made any changes
+    - Include the command you used to run the container
+- What you expected would happen
+- What actually happens
+- Notes (possibly including why you think this might be happening, or stuff you tried that didn't work)
+
+## License
+By contributing, you agree that your contributions will be licensed under its [MIT License](./LICENSE).