extend README.md with build doc

[kindlydodo]

I would like how to correctly setup a local build

[abarisain]

Sorry, but the project is way too old. I tried to get it to build months ago and it just did not.

It would take a lot of time to write a readme :/

[kingosticks]

Maybe you could mark it as archived? https://docs.github.com/en/repositories/archiving-a-github-repository/archiving-repositories

[hurzl]

Or you could try my fork: https://github.com/hurzl/dmix ;-)

[abarisain]

Long time no see 😄

But yeah use their fork.