Move Go code to src/cmd

This does not get everything where we want it finally, see #1266
nor the optimal way of building, but it gets it out of top level.

Added instructions to build if you have a Go installation.

Not moving `vendor` yet.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

src/cmd/moby/main.go

@@ -12,7 +12,7 @@ import (
 	"os/exec"
 	"path/filepath"
 
-	"github.com/docker/moby/pkg/initrd"
+	"github.com/docker/moby/src/initrd"
 )
 
 const (