Add link to Vercel preview

Author: jhildenbiddle

README.md

@@ -9,7 +9,8 @@
 
 Docsify-themeable is a delightfully simple theme system for [docsify.js](//docsify.js.org). Features multiple themes with rich customization options, an improved desktop and mobile experience, and legacy browser support (IE11+).
 
-- [Homepage](https://jhildenbiddle.github.io/docsify-themeable) - Live demo, documentation, and theme previews
+- [Documentation](https://jhildenbiddle.github.io/docsify-themeable) - Demo, installation, options, and theme previews
+- [Development Preview](https://docsify-themeable-preview.vercel.app) - Live preview of [dev](https://github.com/jhildenbiddle/docsify-themeable/tree/dev) branch build
 
 ## Screenshots