Add contribution guide

CONTRIBUTING.md

+use KiCad: http://kicad-pcb.org/
+
+The panelized (3 "subboards" on one board) version needs to be manually updated
+after you have changed the unpanelized version. Copy and paste 3 versions of the
+unpanelized board, adjust the borders (which are milled to separate the boards, 
+but just partially, so that the whole thing is still hanging on the breakable 
+tabs with the small holes ("mice teeth").
+
+Depending on the board manufacturer, you will have to slightly adapt the output
+files, e.g. change filenames and stuff. And you will have to pack it all into
+one .zip file with the right layout. Your board manufacturer will have some 
+README page with the details (hopefully).
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