Update configuration docs section

* Update `subtitle` documentation to note that HTML escape sequences are
restricted by the new Goldmark rendering engine
* Add documentation for the `mainSections` parameter

README.md

@@ -36,9 +36,9 @@ file.
 
 The footer layout file uses Hugo's `markdownify` pipe to display the `copyright` 
 configuration setting, providing support both for HTML character escape
-sequences[^1] such as `©` as well as markdwn formatting and links.
+sequences such as `©` as well as markdwn formatting and links.
 
-[^1] Hugo versions 0.60.0 and up will have restrictions on using embedded HTML, 
+Note: Hugo versions 0.60.0 and up will have restrictions on using embedded HTML, 
 including escape secquences, unless the Goldmark renderer is configure with
 `unsafe = true`.