atben/README-PCB: added manufacturing specification/instructions (in progress)

atben/README-PCB

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+Board characteristics:
+
+- stacking: 2 layers, solder mask on front and back, silk screen on front
+- board material: FR4, thickness 0.8 mm, 1 oz copper
+- surface finish: ENIG (preferred) or tin
+- via holes: diameter is nominally 10 mil, but any size <= 15 mil can be used
+- mechanical tolerances: <= +/- 0.1 mm on all sides
+
+Layer stacking, from top to bottom:
+
+atben-SilkS_Front.gto	Front silk screen
+atben-Mask_Front.gts	Front solder mask
+atben-Front.gtl		Front copper
+atben-Back.gbl		Back copper
+atben-Mask_Back.gbs	Back solder mask
+
+Other design files:
+
+atben-PCB_Edges.gbr	Board edges, for routing (Gerber)
+atben.dxf		idem (AutoCAD DXF)
+atben.drl		Excellon drill file
+
+Interpretation of files:
+
+- do not print PCB edges on front/back copper
+- do not print component values on silk screen
+- the center (!) of the board edge line marks the true board edge, e.g.,
+
+                             Edge line (5 mil)
+                        =======
+         ------- - - - - - ----------
+               |           |
+   PCB outside |           | PCB inside
+               |           |
+         ------- - - - - - ----------
+                     |
+               Volume removed when cutting
+               (width depends on tool used)