Add Play.cash to demos list (preactjs#719)

Author: feross

README.md

@@ -84,6 +84,7 @@ Preact supports modern browsers and IE9+:
 - [**Rainbow Explorer**](https://use-the-platform.com/rainbow-explorer/) _([Github Project](https://github.com/vaneenige/rainbow-explorer/))_ :rainbow:
 - [**Offline Gallery**](https://use-the-platform.com/offline-gallery/) _([Github Project](https://github.com/vaneenige/offline-gallery/))_ :balloon:
 - [**Periodic Weather**](https://use-the-platform.com/periodic-weather/) _([Github Project](https://github.com/vaneenige/periodic-weather/))_ :sunny:
+- [**Play.cash**](https://play.cash) :notes:
 
 ## Libraries & Add-ons