📝 Ajout du CONTRIBUTING.md

Fixes issue zenika-open-source#211

CONTRIBUTING.md

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+# Contributing
+
+Contributions, issues and feature requests are very welcome. If you are using this app and fixed a bug or improve it, please consider submitting a PR!
+
+## Guidelines
+
+### Commit messages
+
+This project uses [gitmoji](https://gitmoji.carloscuesta.me/) as commit convention, because it's fun and provides an easy way of identifying the purpose or intention of a commit with only looking at the emojis used. All you have to do is to find the emoji corresponding to the purpose of your commit in [this list](https://gitmoji.carloscuesta.me/) following by a short message explaining the update. You can also add a description if it's needed. That's all ;)
+
+https://gitmoji.carloscuesta.me/