Add a screenshot placeholder

Author: peggychi

README.md

@@ -15,4 +15,6 @@ See [your demo video title here] (https://link_to_your_video)
 * Hat tip to anyone who's code was used
 * Any other support
 
+<img src="screenshots/main.png" height="400" alt="Screenshot"/>
+
 *Feel free to enhance your README. For Markdown syntax, see [the GitHub Guides](https://guides.github.com/features/mastering-markdown/). Remove this line in your submission.*