Add readme for protoexpress

hardware/ecp5_mainboard/mainboard_prod/readme-protoexpress.txt

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+TrellisBoard rev1.0
+--------------------
+
+Fork Sand, Inc. fork/clone.
+
+Contact: Jeff Moe <moe@forksand.com>
+
+Files:
+ - gerber/: gerbers for all layers
+        F_Cu, In1_Cu..In6_Cu, B_Cu: copper layers from top to bottom (8 layers)
+        F_SilkS, B_SilkS: top and bottom silkscreen
+        F_Mask, B_Mask: top and bottom soldermask
+        F_Paste, B_Paste: top and bottom stencil
+        Edge_Cuts: board outline
+        NPTH: non-plated holes drill
+        PTH: plated holes drill
+ - part_mapping_top.svg, part_mapping_bottom.svg: graphical part mapping (copy of silkscreen)
+ - ecp5_mainboard-all-pos.csv: x & y coordinates for all SMD parts
+ - ecp5_mainboard_bom_royalcircuit.ods: bill-of-materials for all parts in LibreOffice format
+ - ecp5_mainboard_bom_royalcircuit-export.xls: bill-of-materials for all parts exported to XLS format
+
+BOM notes:
+ - Turnkey order, all parts to be ordered by ProtoExpress
+ - Parts with "Substition Allowed" = N must be exact part number match from a distributor (DigiKey, Mouser, etc). DigiKey part numbers are typically given but other distributors may be used
+ - Parts with "Substition Allowed" = Y may be substitued with parts of an equal or better specification from another manufacturer/distributor.
+
+PCB specification:
+ - 200mm x 111.14mm
+ - 8 layers
+ - FR4 material
+ - 1.6mm board thickness
+ - 1oz copper thickness
+ - 3mil track/3mil space
+ - 0.2mm minimum hole diameter
+ - ENIG surface finish
+ - Hard gold fingers with 30 degree bevelling for card edge connector
+ - Green soldermask
+ - White silkscreen
+ - RoHS: Yes