doc: make it clearer when to use the -- form.

agentzh · agentzh · commit 46334b311f32 · 2016-06-30T12:29:48.000-07:00
diff --git a/README.markdown b/README.markdown
@@ -452,6 +452,8 @@ which yields
  -n is an option
 ```
 
+Use this form when you want to output anything leading with a dash (`-`).
+
 [Back to TOC](#table-of-contents)
 
 echo_duplicate
