x/sys/unix: update README.md file to include information on original repo

[programmingkidx]

Go version

go 1.22.0 darwin amd64

Output of go env in your module/workspace:

Not needed

What did you do?

I read the file README.md in the src/cmd/vendor/golang.org/x/sys/unix folder.

What did you see happen?

The program mkall.sh does not work as the README.md file states. There are files missing. See #67178 for more information.

What did you expect to see?

I ask that the information on the upstream repo be included in the file README.md. This is where the upstream repo is located: https://cs.opensource.google/go/x/sys/+/refs/tags/v0.20.0:unix/.

[ianlancetaylor]

The files do exist. You are looking at a vendored copy, not the actual repository.