Add build status badge

README.md

@@ -4,6 +4,7 @@
   <p>A collection of Docker containers for running Blender headless or distributed ✨</p>
   <img alt="Python version" src="https://img.shields.io/badge/python-3.6-blue.svg" />
   <img alt="License" src="https://img.shields.io/badge/License-Apache%202.0-yellow.svg" /><br/>
+  <a href="https://travis-ci.org/nytimes/rd-blender-docker"><img alt="Build status" src="https://travis-ci.org/nytimes/rd-blender-docker.svg?branch=master"></a>
 </div>
 
 ## Getting started