add docs about using bjorn3's much nicer patched version of git subtree and where to get it

[programmerjake]

https://rust-lang.zulipchat.com/#narrow/stream/257879-project-portable-simd/topic/splat.20no.20longer.20compiles.20for.20release.20builds/near/352702343

@bjorn3

[programmerjake]

as mentioned on zulip, turns out the docs are in clippy's docs, however bjorn's script is still much easier to use than the one linked in the docs because it doesn't require any /usr modifications, so I think the docs still need to be updated.

[jieyouxu]

Triage: not sure what the status of this issue is.