Update sample markdown in docs

Author: adamwathan

docs/components/MarkdownSample.mdx

@@ -2,7 +2,15 @@
 
 [View on GitHub](https://github.com/tailwindcss/typography)
 
-This is just a big block of example content designed to dogfood the Tailwind CSS typography plugin.
+A plugin that provides a `rich-text` class you can use to add sensible typographic defaults to any vanilla HTML you don't control (like HTML rendered from Markdown, or pulled from a CMS).
+
+```html
+<div class="rich-text">
+  {{ markdown }}
+</div>
+```
+
+What follows is just a big block of example content designed to dogfood the plugin.
 
 It includes every sensible typographic element I could think of, like **bold text**, unordered lists, ordered lists, code blocks, block quotes, _and even italics_.