add theme preview image

Author: skullface

README.md

@@ -8,6 +8,8 @@ Your personal website is waiting to be personalized, though. It includes space t
 
 It's all possible using the combination of [Jekyll](https://jekyllrb.com/docs/) (for building your website), [GitHub Pages](https://pages.github.com/) (for hosting your website), and [GitHub's API](https://developer.github.com/v3/) (for automatically populating your website with content).
 
+![](https://user-images.githubusercontent.com/221550/110506678-51906280-80cd-11eb-803a-c41984bd9312.png)
+
 ## Installation
 
 ### Fork the `github/personal-website` repo