update readme

Author: ecarriou

README.md

@@ -1,5 +1,7 @@
 # System Designer for Cordova
 
+[![Donate](https://liberapay.com/assets/widgets/donate.svg)](https://en.liberapay.com/ecarriou/donate)
+
 ## Installation
 
 You need to install [Cordova](http://cordova.apache.org) before. Then clone the repository: