Add build status to README.md

Author: mrako

README.md

@@ -4,6 +4,8 @@
 
 When using this tool, you only need to pick the `wait-for` file as part of your project.
 
+[![Build Status](https://travis-ci.org/mrako/wait-for.svg?branch=master)](https://travis-ci.org/mrako/wait-for)
+
 ## Usage
 
 ```