Added contribution guidelines

CONTRIBUTING.md

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+# How to contribute?
+
+## Testing 
+
+Just use OpenTracks and tell us what can be improved by opening an issue [here](https://github.com/OpenTracksApp/OpenTracks/issues/new/choose)
+
+If we broke things (happens now and then), please tell us also.
+
+## Coding new features
+
+You have a cool idea for a feature that is missing in OpenTracks?
+And you would like to help by implementing it?
+If not, you can just take a look at the [issue tracker](https://github.com/OpenTracksApp/OpenTracks/issues).
+
+Helpful hints:
+* creating an issue beforehand might be helpful
+  Sometimes it is helpful to discuss your idea and also your implementation approach first.
+  We may be able to give you some hints on how to tackle the problem.
+  It also makes merging your changes easier and, usually, faster.
+* please only add new dependencies if is absolutely necessary.
+  If you need to:
+  * tell us why those are required,
+  * what alternatives are existing, and
+  * why you choose this dependency.
+  And also verify that the license(s) are compatible with _Apache 2.0_.
+ 
+## Coding conventions
+
+None so far.
+Just keep it simple.
+And follow the _scout rule_. 
+
+You can take a look at existing code and get a feeling for it.
+
+Please autoformat your using AndroidStudio's code formatter.