Add newline before command categories

The simple formatting change adds a nice blank line before each command
category.  Documentation in README.md is also updated to be more
accurate.

help.go

@@ -29,6 +29,7 @@ AUTHOR{{with $length := len .Authors}}{{if ne 1 $length}}S{{end}}{{end}}:
    {{end}}{{$author}}{{end}}{{end}}{{if .VisibleCommands}}
 
 COMMANDS:{{range .VisibleCategories}}{{if .Name}}
+
    {{.Name}}:{{end}}{{range .VisibleCommands}}
      {{join .Names ", "}}{{"\t"}}{{.Usage}}{{end}}{{end}}{{end}}{{if .VisibleFlags}}