doc: Add doc for chunihook

Stolen from various threads from our favorite place

doc/config/chunithm.md

@@ -0,0 +1,46 @@
+# Chunithm configuration settings
+
+This file describes configuration settings specific to Chunithm.
+
+Keyboard binding settings use
+[Virtual-Key Codes](https://docs.microsoft.com/en-us/windows/win32/inputdev/virtual-key-codes).
+
+## `[io3]`
+
+Cabinet specific inputs.
+
+### `test`
+
+Default `0x31` (`1 Key`)
+
+Key-binding for cabinet test button.
+
+### `service`
+
+Default `0x32` (`2 Key`)
+
+Key-binding for cabinet service button.
+
+### `coin`
+
+Default `0x33` (`3 Key`)
+
+Key-binding for cabinet coin switch.
+
+## `[slider]`
+
+Key bindings for each of the 32 touch cells.
+
+### `cellX`
+
+Defaults to key mappings in left-to-right-order as follows:
+
+```text
+SSSSDDDDFFFFGGGGHHHHJJJJKKKKLLLL
+```
+
+Key binding for a single touch cell. Replace the `X` with a value from `1` to `32` to bind keys to
+each cell, e.g. `cell32=0x53`, `cell1=0x53` etc.
+
+Touch cells are numbered FROM RIGHT TO LEFT! starting from 1. This is in order to match the
+numbering used in the operator menu and service manual.