Create CONTRIBUTING.md

CONTRIBUTING.md

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+Thank you for considering contributing to the [imperialism remake](http://remake.twelvepm.de/)
+open source game project!
+
+Volunteers are always welcome. There are plenty of chances to take part in coding,
+graphics design, discussing the game rules or just giving feedback.
+
+Most of these tasks can be discussed in the [community forum](http://remake.twelvepm.de/forum/),
+please consider registering there.
+
+For programmers direct interaction with the [github repository](https://github.com/Trilarion/imperialism-remake)
+by forking and pushing merge requests are preferred. Please use the
+[issues tracker](https://github.com/Trilarion/imperialism-remake/issues) to report problems with
+the game. Requests for enhancement should be posted in the community forum or if they more technical
+(coding related) also at the issues tracker.