Update README to reflect new Go build process

[asteele4]

I don't work with Go but would like to build this.

The description of how to build it seem to be out of date and apply to older versions of Go.

Could someone with more knowledge than myself update the README to show how to build using the current version of Go.

[Idriel]

Hi, don't know is this still relevant. But it's lacking go.mod for new versions.

You can create it like:
$ go mod init github.com/uber/pam-ussh
$ go mod tidy
// resolve needed library
$ sudo apt-get install libpam0g-dev

now make will work.