installation instructions are now out of date

I don't know anything about `go`, never mind `go get`, but when I installed the latest version `1.18` inside WSL2, I was alerted to use `go install` instead of `go get`. So, what I ended up having to do to use this to pipe through windows: ```bash GOOS=windows go install github.com/jstarks/npiperelay@latest ``` For future people who land here: you'll want to check your GOPATH via ```bash go env GOPATH ``` and then make sure that `npiperelay.exe` is in your `GOBIN` -- which might be something like `/home/<USER>/go/bin/`. I then ended up having to setup a symlink as so ```bash sudo ln -s /home/<USER>/go/bin/windows_amd64/npiperelay.exe /usr/local/bin/npiperelay.exe ``` there was no need to link this back to my windows user.