Add building instructions

[cibersheep]

Hello. Thanks for the code :)

It may be nice to have a small read me with the building instructions.
I was able to build on Ubuntu 20.04 with cmake && make but when tried to build with snapcraft I go this error:

archetype/drivers$ snapcraft 
Launching a VM.
Updating pull step for archetype (source changed)
+ snapcraftctl pull
Building archetype 
+ snapcraftctl build
make -j2
make: /usr/bin/cmake: Command not found
Makefile:1276: recipe for target 'cmake_check_build_system' failed
make: *** [cmake_check_build_system] Error 127
Failed to run 'make -j2' for 'archetype': Exited with code 2.
Verify that the part is using the correct parameters and try again.