Create CONTRIBUTING.md

CONTRIBUTING.md

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+It is a place to share ideas on what to implement, gather people that want to work on the engine,
+and work on the source code.
+
+If you find a bug or have an enhancement request, file an [Issue](https://github.com/openxray/xray-16/issues).
+
+Pull requests appreciated! However, the following things should be taken into consideration:
+* We want to keep the game as close as possible to the vanilla game, so instead of introducing new gameplay features,
+  consider adding non-gameplay features, fixing bugs, improving performance and code quality
+* Major changes should be discussed before implementation
+* Follow the [procedures](doc/procedure)