Add button screenshot to README

Shows the 'Open Markdown' dropdown UI in documentation.
Screenshot stored in .github/images/ (excluded from npm package).

Author: FlyNumber

.github/images/button_screenshot.png

@@ -19,6 +19,10 @@ A lightweight Docusaurus plugin that exposes your markdown files as raw `.md` UR
 
 See it in action at [flynumber.com/docs/](https://www.flynumber.com/docs/) - try clicking the "Open Markdown" dropdown next to any page title!
 
+## Screenshot
+
+![Open Markdown Dropdown](.github/images/button_screenshot.png)
+
 ## Installation
 
 ```bash