Added section explaining Markdown support

The inline markdown via `markdown:` is one my favorite features. I'm not certain if it exists in all uses of Slim, but I haven't seen otherwise.

slim.md

@@ -60,6 +60,18 @@ ruby:
 div= foobar
 ```
 
+### Inline Markdown
+
+```jade
+markdown:
+  ### On Markdown
+  
+  Slim can handle your [Markdown](https://daringfireball.net/projects/markdown/syntax) content for longer content blocks or `code`.
+  
+  Depending on your parser, like [Kramdown](https://kramdown.gettalong.org/quickref.html), other features might work, like assigning attributes or classes.
+  {: .classname}
+```
+
 ### Embedded JavaScript
 
 ```jade
@@ -67,6 +79,7 @@ javascript:
   alert('Slim supports embedded javascript!')
 ```
 
+
 ### Comments
 
 ```jade