Mention go install method in README.

[apapsch]

Installing via go get in module mode produces the deprecation message:

go get: installing executables with 'go get' in module mode is deprecated.
        Use 'go install pkg@version' instead.
        For more information, see https://golang.org/doc/go-get-install-deprecation
        or run 'go help get' or 'go help install'.