Note the front matter requirement in pages.md (#9423)

Merge pull request 9423

docs/_docs/pages.md

@@ -9,7 +9,7 @@ members or recipes).
 
 The simplest way of adding a page is to add an HTML file in the root
 directory with a suitable filename. You can also write a page in Markdown using
-a `.md` extension which converts to HTML on build. For a site with
+a `.md` extension and front matter which converts to HTML on build. For a site with
 a homepage, an about page, and a contact page, here’s what the root directory
 and associated URLs might look like: