initial check-in of bgt-keystone project

This adds a 3U component carrier module into which you can mix and
match keystone modules as needed.

bgt-keystone/README.md

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+# 3U component carrier module for 6 keystone connectors
+
+This is a mechanical design to mount 6 keystone connectors in a 3U
+component carrier.
+
+Why would one want to do that?  You can get keystone modules
+inexpensively for virtually any [small] connector, like RJ45, RJ12, BNC,
+SMA, LC, USB-A, USB-B, ...
+
+You can configure a keystone panel dynamically and quickly for the given
+use case of connector combinations.  No need to do a custom face plate
+each time, so it's particularly useful in setups you're building once,
+where the design cost, machine setup cost and production delay of a
+custom face plate are prohibitive.
+
+The module consists of two panels:
+
+* the face plate to which the keystone modules are flush
+* the back plate into which you snap the keystone modules
+
+## face plate
+
+6 TE wide face plate
+
+* 128.4 x 30.22 mm
+* 14 openings
+* 907 mm total path
+* 2mm (2.5mm) material, typically AlMg3
+
+
+## back plate
+
+* 110 x 29 mm
+* 12 openings
+* 763 mm total path
+* 1mm material, typically steel
+
+
+## BOM
+
+* 1x face plate (above)
+* 1x back plate (above)
+* 6x 4mm hexagon spacers with 2mm thread
+* 6x M2 countersunk screws (from front side)
+* 6x M2 screws (from back side)
+* 2x Fischer KN 1
+* 2x Fischer HS 1 or RS 1 M2,5x11