Add CONTRIBUTING (based on 2FAuth-Docs contributing page)

.github/CONTRIBUTING.md

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+# Contributing
+
+2FAuth is an Open Source project, that means everybody can participate, in many differents ways.  
+If Open Source is new to you, the following guide should be a good starting point:
+
+🔗 [How to Contribute to Open Source](https://opensource.guide/how-to-contribute/)
+
+## Enhancement & Feature suggestion
+
+Feel free to request some enhancements or new features. Not every request will be accepted but it can always be discussed.
+
+Also please have a look to the 2FAuth development project, maybe your idea is already there.
+
+💡 [2FAuth Backlog project on GitHub](https://github.com/users/Bubka/projects/1)
+
+💬 [2FAuth discussion on GitHub](https://github.com/Bubka/2FAuth/discussions)
+
+## Development
+
+2FAuth is made with the following technologies:
+
+- PHP
+- Javascript
+- HTML/CSS
+- SQL* databases
+
+If you are confortable with one of this language you are very welcome to submit some additions,  modifications or fixes to the code base via a Pull Request on <a href="https://github.com/">GitHub</a>.
+
+> ⚠️ Please make your PR on the `dev` branch
+
+## Translation
+
+2FAuth is localized in several languages. You can contribute by correcting or completing translations in a language you speak, using the Crowdin platform.
+
+Ask for your language if this does not yet exist.
+
+🔗 [2FAuth translations on Crowdin](https://crowdin.com/project/2fauth)