doc: Add iidxio-bio2 documentation

doc/iidxhook/iidxio-bio2.md

@@ -0,0 +1,18 @@
+# IIDXIO API implementation with BIO2 driver
+This implementation of BT5's iidxio API allows you to use native BIO2 hardware with anything that
+supports BT5's iidxio API, e.g. all iidxhooks of BT5. Thus, you can play your favorite old IIDX
+games with the latest generation of hardware.5.
+
+## Setup
+* Have `iidxio-bio2.dll` in the same folder as your `iidxhookX.dll`
+* Rename `iidxio-bio2.dll` to `iidxio.dll`
+* Ensure that your `gamestart.bat` actually injects the appropriate iidxhook dll, for example:
+```bat
+inject iidxhook3.dll bm2dx.exe ...*
+```
+or
+```bat
+launcher -K iidxhook4.dll bm2dx.dll ...*
+```
+* This assumes that the BIO2 is already flashed to the correct firmware. The firmware perists once
+flashed and does not need to be re-flashed after a power cycle.