Add a linux/arm release

[leoadberg]

Would be pretty handy for me and presumably others (I'm trying to run it on old Raspberry Pi). Cross compiling from my main computer (Mac) is a pain because of CGO and compiling pixlet takes literal hours on my underpowered Pi. Thanks!

[mattbroussard]

According to this comment there is Linux arm64 support now: #38 (comment)

For 32 bit building for Raspberry Pi, I was able to follow this guide (though it would be nice to distribute a 32bit ARM binary too since running on a Raspberry Pi is a common enough use case): https://github.com/drudge/homebridge-tidbyt/wiki#how-do-you-install-pixlet-on-the-raspberry-pi