Add instructions on how to build to the README. #213

Job	Run time
x86_64 appimage	7m 14s
i386 appimage	7m 10s
aarch64 appimage	10m 19s
armhf appimage	9m 36s
x86_64 coverage	6m 4s
Create release and upload artifacts	11s
	40m 34s

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