konami/iidx: Add instructions for building turn table bypass cable

konami/product/GULDJ-JS.md

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+# `GULDJ-JS`: Beatmania IIDX Rootage art and turntable bypass kit
+
+## Turntable bypass cable
+
+If you upgraded a [C02 wired cabinet](../io.md#c02-io), the turntables are connected to the
+sub IO via a serial line that goes to another sub IO behind the front panel
+next to the 16seg display. This introduces additional latency as the input
+data needs to pass through multiple systems/PCBs until it reaches the game.
+
+This can be reduced by using a so-called "bypass" cable, that connects the
+turntables directly to the [BIO2 sub IO](../io.md#bio2-ldj-sub-io) board by emulating the wiring of a
+[D01 IO](../io.md#d01-io) based cabinet, reducing latency significantly.
+
+This cable could be ordered with the IIDX Rootage upgrade kit but can also
+be easily created from scratch.
+
+### Parts
+
+* Some wires
+* [JST HL](https://jst.de/file/download/219/pitch-3-96-mm-wtw-hl2-pdf) 2x4 pin female connector
+* [JST HVQ](https://jst.de/file/download/637/hvq) 8 pin female connector
+* Matching terminals for the connectors
+
+### Pinout
+
+```text
+JST HL 2x4 pin front view connector
+
+notch at top
+    ___
+|---------|
+| 1 2 3 4 |
+| 5 6 7 8 |
+|---------|
+```
+
+```text
+JST HVQ 8 pin front view connector
+Numbering on connector matches
+
+   notch at top
+___________________
+-------------------
+|-----------------|
+| 1 2 3 4 5 6 7 8 |
+|-----------------|
+
+```
+
+Pins of the two connectors need to be connected like this:
+| JST HL 2x4 pin | JST 8 pin |
+|----------------|-----------|
+| 1              | 1         |
+| 2              | 1         |
+| 3              | 3         |
+| 4              | 2         |
+| 5              | 5         |
+| 6              | 4         |
+| 7              | 8         |
+| 8              | 8         |