readme: "go get" no longer how to install a command

[jowagner]

Following the "getting started" instructions in the readme, I get:

$ go get github.com/google/skicka
go: go.mod file not found in current directory or any parent directory.
        'go get' is no longer supported outside a module.
        To build and install a command, use 'go install' with a version,
        like 'go install example.com/cmd@latest'
        For more information, see https://golang.org/doc/go-get-install-deprecation
        or run 'go help get' or 'go help install'.

Adjusting to

go install github.com/google/skicka@latest

I got a skicka binary in ~/go/bin