Add mention of starry-night to readme (#102)

Author: wooorm

readme.md

@@ -44,7 +44,7 @@ Import the `github-markdown.css` file and add a `markdown-body` class to the con
 </article>
 ```
 
-If you want code syntax highlighted, use GitHub Flavored Markdown rendered from [GitHub's `/markdown` API](https://docs.github.com/en/free-pro-team@latest/rest/reference/markdown).
+You can use [GitHub's `/markdown` API](https://docs.github.com/en/free-pro-team@latest/rest/reference/markdown) to turn Markdown into the HTML that GitHub generates, which works well with the CSS in this repo. Other Markdown parsers will mostly work with these styles too. To mimic how GitHub highlights code, you can use [`starry-night`](https://github.com/wooorm/starry-night) with your Markdown parser of choice.
 
 There are 3 themes provided in this package: