Add pi-mqtt-gpio project with gpiod feature branch

Uses a public github fork

Add pi-mqtt-gpio project with gpiod feature branch
c8622759 · Alexander Wellbrock · a40eb06f · c8622759 · ac94f732
Verified Commit c8622759 authored 5 years ago by Alexander Wellbrock
--- a/.gitmodules
+++ b/.gitmodules
+[submodule "pi-mqtt-gpio"]
+	path = pi-mqtt-gpio
+	url = git@github.com:w4tsn/pi-mqtt-gpio.git
--- a/pi-mqtt-gpio @ ac94f732
+++ b/pi-mqtt-gpio @ ac94f732
+Subproject commit ac94f732da5e20baa7cdbfd0966ca1084513da76