Update README.md

Add download app badge from SourceForge

Author: spinopel

README.md

@@ -3,6 +3,7 @@
 
 ![Static Badge](https://img.shields.io/badge/status-experimental-orange?labelColor=yhjgjhj)
 [![Maintainability](https://api.codeclimate.com/v1/badges/68b6262dfd482dc7891a/maintainability)](https://codeclimate.com/github/spinopel/webapp-android-webview/maintainability)
+[![Download webapp-android-webview](https://img.shields.io/sourceforge/dt/webapp-android.svg)](https://sourceforge.net/projects/webapp-android/files/latest/download)
 
 Simple build your website into a Android application.