Add sample usage of meta frontmatter to layout

Signed-off-by: Matthew Peveler <matt.peveler@gmail.com>

source/index.html.md

@@ -17,6 +17,10 @@ includes:
 search: true
 
 code_clipboard: true
+
+meta:
+  - name: description
+    content: Documentation for the Kittn API
 ---
 
 # Introduction