Add CONTRIBUTING.md

[dragetd]

Issue to track progress on writing a CONTRIBUTING.md :-)
A rough, initial plan has been written by ubitux already in #2 which was:

Here is a quick & dirty howto:

• create boards/.scad following other .scad files in the same directory. This .scad is directly open-able with openscad.
• if necessary, add the electronic components in electronics/.scad. The .scad files are directly open-able with openscad.
• register the board in boards.scad (include of boards/.scad, mapping array to the info function, and the 2 modules entries)
• create cases/.scad following other .scad files in the same directory (this one should be pretty simple). This .scad is directly open-able with openscad.
• if your board has button pushers, you want to register it in cases/Makefile (BOARDS_WITH_BUTTON_PUSHERS)
• make -j$(nproc) gif will generate the gif (it may take some time)
• edit the .md files to add the necessary entries (board, cases, electronics).
  This step could be automatized, but it lacks a description to extract from somewhere. Maybe a // @desc or whatever in the .scad files or whatever could do the trick (extracted with a grep or whatever).