Add demo app screenshot

Author: gonzalezreal

Demo/Screenshot.png

@@ -178,6 +178,8 @@ fully complies with the [CommonMark Spec](https://spec.commonmark.org/current/).
 Below you can see a few examples of how MarkdownUI renders Markdown elements. Additionally, you can
 explore the full MarkdownUI capabilities in the [companion demo project](Examples/MarkdownUIDemo).
 
+![Demo app screenshot](Demo/Screenshot.png)
+
 ### Block Quotes
 
 ```swift