Add instructions for running the Rust build locally

[vthemelis]

The command here:

https://github.com/facebook/errpy/blob/main/README.md?plain=1#L46

fails due to ocaml/opam#3460

I locally worked around it by modifying the opam sandbox script but there may be a better solution to this.

Could we add instructions on how to bypass the issue or add the necessary config?