refactored CONTRIBUTING.md

Author: Martin Plieske

CONTRIBUTING.md

@@ -1 +1,8 @@
-If you want to contribute, use the "JavaFX Scene Builder 8.5.0" for fxml files.
+#Contributing
+
+If you want to contribute please follow this guideline.
+
+##FXML files
+
+For editing fxml files, use the "JavaFX Scene Builder" in version 8.5.0.  
+Layouting is generally done with fxml files.