Document arch/OS support expectations

[workingjubilee]

We have some things that are explicitly supported, some things that are explicitly unsupported, and some things that are known to not work but we are interested in exploring, or are unknown to work and if you can use them... then... sure?

My current understanding:

• 32-bit architectures are "explicitly unsupported" at the current moment, but we don't compile_error! currently because last we heard, someone was poking around with this possibility. We haven't heard back recently, so maybe we should add in a compile_error!
• Windows is ???: we aren't able to offer much technical expertise from our current bank of contributors, but we aren't all that averse to increasing support for it.
• We expect a typical Linux distro, with our prescribed configs, to essentially always work.
• We expect macOS to work, but homebrew sometimes is painful enough to make that a "usually".
• We don't explicitly support non-Linux non-Apple Unix OS, but they tend to work.