Added a cover sheet to separate counterweight and main PCB.

- README: describe mechanical stacking and the function of the various
  protective layers
- cvr.py: model of the cover sheet, with gnuplot and HeeksCAD output

README

@@ -0,0 +1,30 @@
+Mechanical stacking
+-------------------
+
+From the bottom to the top, we have the following elements:
+
+- Ben case, bottom shell
+- a few drops of glue or silicone, to hold the counterweight in place
+- the counterweight, covered by protective paint
+- a few drops of glue or silicone, to keep the cover sheet in place
+- a cover sheet of thin hard plastic, e.g., the type of plastic film used to
+  make transparencies
+- isolating tape, applied to tall components of the Ben's main PCB
+- the Ben's main PCB
+
+
+Protection
+----------
+
+The counterweight is covered by one or more layers of paint, to prevent
+direct skin contact with the lead during handling. The paint may also
+offer some amount of protection against electrical contact.
+
+The counterweight is covered by a layer of hard plastic that isolates
+from electrical contact and that also resists being punctured by pointy
+components or solder joints of the main PCB.
+
+Finally, all elements on the main PCB that are unusually tall are taped
+over, to further reduce the risk of them working their way into the
+counterweight. Right now, the only component where problems are
+considered likely is the buzzer.