add build documentation to README.md

[bitrot-alpha]

Pull Request checklist

Note: these are not necessarily requirements

• [ ] I reformatted the code with clang-format
• [x] I checked to make sure my submission worked
• [x] I am the author of submission or have permission from the original author
• [ ] Requires update of the PS2SDK or other dependencies
• [ ] Others (please specify below)

Pull Request description

Add build instructions (for Linux anyway) Meson's website is too verbose. I wanted "how the hell do I get my executable" but there were no explicit instructions so I've written out what I managed to figure out. Hopefully it's correct/helps someone.