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

@@ -35,16 +35,34 @@ As a very basic theme, there is very little to be configured via the `config.tom
 file.
 
 The footer layout file uses Hugo's `markdownify` pipe to display the `copyright` 
-configuration setting, providing support both for HTML5 character escape
-sequences such as `©` as well as markdwn formatting and links.
+configuration setting, providing support both for HTML character escape
+sequences[^1] such as `©` as well as markdwn formatting and links.
 
-The only non-standard configuration setting is the `subtitle` parameter:
+[^1] 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`.
+
+### `subtitle` parameter
+
+Setting the `subtitle` parameter adds a secondary header underneath the `title`
+in the site header.
 
 ```toml
 [params]
   subtitle = "A sub-title for your site"
 ```
 
+### `mainSections` parameter
+
+Setting the `mainSections` parameter restricts the pages that appear on the
+homepage.
+
+```toml
+[params]
+  mainSections = ["posts"]
+
+```
+
 ## Theme Organization
 
 ### Semantic Content Organization