Add support for long help strings for child commands

[fgrehm]

@cv looks like it is only supported for top level commands READMEs:

sd/cli/cli.go

Lines 195 to 201 in 594ccd5

	if err == nil {
	logrus.Debug("Found README at: ", readmePath)
	cmd.Short = strings.Split(string(readme), "\n")[0]
	cmd.Long = string(readme)
	cmd.Args = cobra.NoArgs
	cmd.RunE = showUsage
	}

I think we'd need something smart to handle this here:

sd/cli/cli.go

Lines 304 to 312 in 594ccd5

	cmd := &cobra.Command{
	Use: filepath.Base(path),
	Short: shortDesc,
	Example: example,
	Annotations: map[string]string{
	"Source": path,
	},
	RunE: execCommand,
	}