Add contribution guide

CONTRIBUTING.md

+To contribute to 3drag powerPWM just open an account on https://git.nexlab.net, 
+fork the project, do your modifications and then open a merge request to the main project.
+
+You can of course also contributing by open issues on the project at https://git.nexlab.net/nexlab/3drag_powerpwm, 
+or even by donating on the project web site at https://www.nexlab.net
+
+Enjoy!
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