chore: update README

README.md

@@ -1,3 +1,30 @@
 # presidium-layouts-base
 Base Hugo layouts for Presidium
 
+## Custom Output Formats
+
+### Snippets
+
+The `Snippet` output format is a alternative HTML output format to the standard output  (`index.html`) for _pages_ and _sections_. It's intended for generating standalone articles that can be used in third-party/isolated contexts (such as in an `iframe`).
+
+Features of a snippet:
+ - Generates content from the `content/` directory.
+ - Uses the same stylesheets as the standard output.
+ - Excludes inter-host navigation contexts that are in the standard output, such as the toolbar and left sidebar.
+ - Includes breadcrumbs to ancestor snippets for limited navigation within the docset.
+
+**Usage**  
+
+Snippet outputs must be configured per docset repository in `config.yml`.
+
+```yaml
+# <my-docset>/config.yml
+...
+outputs:
+  page:
+    - html      # -> index.html
+    - snippet   # -> snippet.html
+  section:
+    - html      # -> index.html
+    - snippet   # -> snippet.html
+```