Create CONTRIBUTING.md

Author: s4cha

CONTRIBUTING.md

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+First off, thank YOU!
+
+It's thanks to people like you that the project thrives :)
+If you are reading this, it means you want to contribute to this project one way or another.
+
+Our vision for Then is to simplify swift async code on all platforms running Swift code.
+
+We are very open to contributions and any ideas that can improve the project !
+
+## How can I contribute?
+
+- Report a bug or strange behavior via an issue
+- Suggest a new feature via an issue
+- Open a pull request
+
+In order to maximize your pull request approval, you may want to open an issue first.
+That way the addition or change can be discussed, so that you don't waste time on a feature that might be rejected later.
+
+
+When you issue a change, make sure you run the unit tests (Cmd+U) and check all pass.
+
+When adding a new feature, it's best if you can add the corresponding unit tests. If this is new to you, no problem, the team can help you
+or add the unit tests for you :)