Add documentation for content-menu block

README.md

@@ -163,6 +163,12 @@ organization. The [list](layouts/_default/list.html) and
 [single](layouts/_default/single.html) templates display the page title as 
 a `<h1>...</h1>` level header.
 
+#### content-menu
+
+This block is for providing navigation menu(s) for the main content, matching
+with the `<nav>...</nav>` block inside the main header. The block is currently
+a place holder, with no default implementation.
+
 #### content
 
 This block is for the content of the page.