Added UI preview to read me file.

README.md

@@ -2,4 +2,6 @@
 
 An installable progressive web app to help with barbell math at the gym. Built this in 2018 when I first started learning HTML, CSS, and JavaScript.
 
+![UI preview of the Lift It app.](/images/What-Are-You-Lifting-App-UI-Preview.png)
+
 [View App](https://stevenhaskell.github.io/WhatAreYouLiftingApp/)