📝 Update README.md with contributing section (#16)

README.md

@@ -1,12 +1,16 @@
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-# Mediateur
+# mediateur
 
 A simple and unambitious library to implement the Mediator design pattern
 
+## Contributing
+
+Before you contribute, please take a few minutes to read the
+[contribution guidelines](https://github.com/spontoreau/mediateur/wiki/Contribution-guidelines).
+
 ## Code of Conduct
 
 This project has adopted the [code of conduct](https://github.com/spontoreau/mediateur/wiki/Code-of-conduct) defined by