Add ability to configure GPIO MIDI baud rate

README.md

@@ -22,6 +22,7 @@ Turn your Raspberry Pi into a dedicated emulation of the [famous multi-timbre so
   * [GPIO MIDI interface](#gpio-midi-interface)
     + [Schematic](#schematic)
     + [Breadboard example](#breadboard-example)
+    + [Serial ports](#serial-ports)
 - [🔊 I2S DAC support](#-i2s-dac-support)
   * [Setup](#setup)
   * [Compatibility](#compatibility-1)
@@ -113,6 +114,12 @@ You can build a simple circuit based on an opto-isolator, a diode, and a few res
 #### Breadboard example
 ![](docs/gpio_midi_bb.svg)
 
+#### Serial ports 
+
+You can also skip the MIDI circuitry and drive `mt32-pi` using a serial port directly using software such as [Hairless MIDI] or [SoftMPU] and suitable cabling. Use the `gpio_baud_rate` option in the configuration file to match up `mt32-pi` with your host's serial port baud rate.
+
+> ⚠️ **Note:** Remember that the Raspberry Pi expects no more than 3.3V on its GPIO pins, so make sure that you use appropriate level shifting when interfacing with other hardware to prevent damage to the Pi.
+
 ## 🔊 I2S DAC support
 
 The Raspberry Pi's headphone jack is a simple PWM device, and not designed for high-fidelity audio. This becomes very obvious when you use `mt32-pi` - distortion in the sound is apparent when quieter sounds are playing.
@@ -291,6 +298,7 @@ This project, just like [Munt], has no affiliation with Roland Corporation. Use
 [Circle]: https://github.com/rsta2/circle
 [digital-to-analog converter]: https://en.wikipedia.org/wiki/Digital-to-analog_converter
 [GY-PCM5102]: https://www.aliexpress.com/item/4000049720221.html
+[Hairless MIDI]: https://projectgus.github.io/hairless-midiserial/
 [I2C backpack]: https://www.adafruit.com/product/292
 [inih]: https://github.com/benhoyt/inih
 [Munt]: https://github.com/munt/munt
@@ -299,3 +307,4 @@ This project, just like [Munt], has no affiliation with Roland Corporation. Use
 [PI-MIDI]: https://www.arananet.net/pedidos/product/pi-midi-a-baremetal-mt32-emulator-using-raspberry-pi3
 [REC002004B]: https://www.raystar-optronics.com/oled-character-display-module/REC002004B.html
 [Releases]: https://github.com/dwhinham/mt32-pi/releases/latest
+[SoftMPU]: http://bjt42.github.io/softmpu/