adding contribution rules

Author: tibbi

CONTRIBUTING.md

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+### Reporting
+Before you report a new issue, make sure you look at the existing ones first to avoid creating duplicates. You can use Search with some keywords too.
+
+### Contributing as a developer
+Some instructions about code style and everything that has to be done to increase the change of your code getting accepted can be found at the [General Discussion](https://github.com/SimpleMobileTools/General-Discussion#contribution-rules-for-developers) section. 
+
+### Contributing as a non developer
+In case you just want to for example improve a translation, you can find the way of doing it [here](https://github.com/SimpleMobileTools/General-Discussion#how-can-i-suggest-an-edit-to-a-file).