Add section in README for building gor from source

[thensgens]

Just a minor proposal here, but you could give a "hint" how to build gor from source (only possible of course if the person has a working go installation).
So, something like "go get ...gor" and maybe note which dependencies are used/required (if any).

Imho it's useful, especially for new Go programmers or to-be Go programmers.