Add a Cross.toml file and a note on using it

[awilkins]

This provides a statically linked build that will work on musl-based
Linuxes such as those commonly used on NAS boxes.

[andrewchambers]

So to clarify - this does the build in docker?

[awilkins]

Yes, cross-rs does the build in Docker and leaves the outputs in the expected place in the host filesystem - I've not attempted to wire it into the existing Github Actions build stuff that someone else contributed.

The cross-rs guys had some more advice like using a lighter build setup (apparently the full Rust / Alpine image is heavier than necessary), but it Works For Me™ as-is ; I got a statically linked bupstash executable that happily runs on my x86 based NAS box that I can just scp into place.