docs(markdown): Document approaches to Markdown theming (#647)

Fixes #550

docs/THEMING.md

@@ -22,3 +22,11 @@ or an array of [vinyl](https://github.com/gulpjs/vinyl) `File` objects.
 The theme is free to implement HTML generation however it chooses. See
 [the default theme](https://github.com/documentationjs/documentation-theme-default/)
 for some ideas.
+
+### Theming Markdown
+
+The default Markdown generator for documentation.js isn't customizable - instead
+of a plain-text theme, it's generated by creating an AST and then rendering
+it with [remark](http://remark.js.org/). If you need something extra in Markdown,
+you can either rally for that thing to be included in the default theme,
+or you can hack around it by using an HTML theme that outputs Markdown.