Add CONTRIBUTING.md

Author: Coderberg

CONTRIBUTING.md

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+Contributing
+============
+
+ResidenceCMS is an open source project. Contributions made by
+the community are welcome. Send us your ideas, code reviews, pull requests and
+feature requests to help us improve this project. All contributions must follow the [usual Symfony contribution requirements](https://symfony.com/doc/current/contributing/index.html).
+
+## Any contributions you make will be under the MIT Software License
+In short, when you submit code changes, your submissions are understood to be under the same [MIT License](http://choosealicense.com/licenses/mit/) that covers the project. Feel free to contact the maintainers if that's a concern.