Add contributing md

Author: patrickfav

CONTRIBUTING.md

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+# Contributing to Bytes
+
+We ❤ pull requests from everyone.
+
+If possible proof features and bugfixes with unit tests.
+This repo validates against checkstyle (import the xml found in the root to your IDE if possible)
+
+To run the tests (and checkstyle):
+
+```shell
+mvn test checkstyle:check
+```
+
+Tests are automatically run against branches and pull requests
+via TravisCI, so you can also depend on that.