Update doc: use `go install` instead

[Richard Zak]

Go no longer uses go get to install a binary from a project:

go get github.com/cloudflare/gokey/cmd/gokey
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'.

Instead, use go install with a version or @latest:

$ go install github.com/cloudflare/gokey/cmd/gokey@latest
go: downloading github.com/cloudflare/gokey v0.1.0
go: downloading golang.org/x/crypto v0.0.0-20190701094942-4def268fd1a4
go: downloading golang.org/x/sys v0.0.0-20190626221950-04f50cda93cb
$ which gokey
/home/user/go/bin/gokey