Added CONTRIBUTING guidelines

Author: mantognini

CONTRIBUTING

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+Contribution Guidelines
+=======================
+
+You would like to see a feature implemented or a bug fixed in SFML? Great!
+Contributions to SFML are highly appreciated, be it in the form of general
+ideas, concrete suggestions or code patches.
+
+A few guiding rules have been set up on the SFML website that you should be
+aware of before opening an Issue or Pull Request. They will help you focus
+on the important stuff and prevent you from losing (y)our time with requests
+that are out of SFML's scope, known issues, and so on.
+
+                http://www.sfml-dev.org/contribute.php
+
+Those rules cover the general scope defined for this project, a coding
+style, and a precise procedure to report bugs or suggest new features.