Add a "how-to-contribute" stub to the root README

Author: gluap

README.md

@@ -23,3 +23,20 @@ If in doubt, go for this one.
 
 ### Permanenty Mounted
 
+# Making/Contributing your own changes
+The files for 3D Printing are in the ``.stl`` format. But these are discretized versions of more universal
+3D Geometry that can be read and edited by CAD software.
+
+This is a grown project and in the past contributions have been made with different CAD Programs. We would
+love to go full Open Source and use Freecad and OpenSCAD exclusively instead of commercial solutions, but a lot of the
+case design is still done in software by the big players. Some parts were designed/modified in Autodesk inventor
+(filenames ``*.ipt``), some were designed in Fusion360 (filenames ``*.f3d``), but for most parts a step file
+(``*.stp, *.step``) is available which can be opened by most CAD Programs. For your contributions in CAD software
+of your choice it would be great if you could also follow the practice of submitting your changes in both
+``.stl`` for 3D-Printing and ``.step`` for later editing - and maybe the native format of your CAD software on
+top.
+
+OpenSCAD is a special case of course: Here submitting ``.stl`` and the ``.scad`` file is sufficient. 
+
+
+