Update contributing tips and add copyright info.

Author: nitrocaster

README.md

@@ -15,6 +15,11 @@ Build instructions: [doc/howto/build.txt](doc/howto/build.txt)
 
 If you find a bug or have an enhancement request, file an [Issue](https://github.com/openxray/xray-16/issues).
 
-Pull requests appreciated! Just check out 
-[the task list](https://github.com/openxray/xray-16/blob/master/doc/design/task_list.txt) 
-and follow our [procedures](https://github.com/OpenXRay/xray-16/tree/master/doc/procedure).
+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 fixing bugs, improving performance and code quality
+* Major changes should be discussed before implementation
+* Follow the [procedures](doc/procedure)
+
+Be advised that this project is not sanctioned by GSC Game World in any way – and they remain the copyright holders
+of all the original source code.